The uses hidden images on bank notes are similar to propaganda notes. The usual reason such hidden images are used covertly is to avoid the very real danger of imprisonment, torture, and/or death. The main goals of hidden messages are:

To bolster the will of their people. Hidden messages predicting the return of the central government or the United States army served to raise the spirits of the Chinese people.

Slander the opponent. A crafty engraver slipping in an obscene gesture, pointed at the occupying country (Japan) had a powerful influence on the Chinese during WWII as well.

A protest to a perceived wrong doing. This is best illustrated by the Ghoul notes of post WWI Germany. See this section below.

• Act as propaganda to attempt to change public attitudes, either foreign or domestic. The 1989 "democracy note" from Burma is such an effort to support democracy efforts.

China Examples

By far, China has the most "hidden images" known at this time. Since Japan occupied China for the duration of much of World War II, the enterprising engravers chose to protest with the real threat of being executed if caught.

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5 chiao notes (50 Cents) with the Mausoleum of Sun Yat-sen on the front contain the letters SGWRS (Central Government Will Return Soon) in the scrollwork above & below the word "Reserve" on the back. The same meaning with five Chinese characters appear on the face; two in the bush to the left of the stairs, one on the left on the building façade under the roof, one above the trees to the right of the building, and one in the bead at the upper right of the conversion clause. The notes were printed by Watson Printing Company, also know as Hua Sheng yinshua youxian gongsi.These notes are:
Pick J5
Pick J6
Pick J7
Pick J18
Here's an example.
Here's a closeup showing the location of the letters.

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Confucius is seen with an obscene gesture in the way his hands are arranged on several notes. These include:
Pick J61, 1 Yuan, 1938
Pick J61, 1 Yuan - Close-Up of Confucius portrait.
Pick J54, 1 Yuan, 1938
Pick J55, 1 Dollar, 1938. A weakly printed lithographed copy like J54 - could be counterfeit
Pick J72, 1 Yuan

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The designs in the right and left margins resemble bisected turtles which were supposed to be insulting to the Japanese on the following note:
Bisected turtles, an animal held in low esteem in China, can be seen in the border of another Central Reserve Bank of China note.
Pick J12, 10 Yuan, 1940

Here's a closeup of the turtles from another example, Pick J63
Pick J63, 10 Yuan, 1938

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Printed in 1944 with a propaganda message hidden in the plate by the engraver Chung Kue-jen. The message, USAC 1945, meant "The U.S. Army is Coming in 1945". This note is:
Pick J30, 200 Yuan Central Reserve Bank of China

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Germany Examples

Historians will tell you the main cause of WWII was WWI. With respect to Germany's developments after WWI this is especially true. Hitler came to power promising to avenge the loss of the war and the conditions of the armistice. One particular sore spot was the Alsace and Lorraine areas annexed to France from Germany. It is no coincidence that the Alsace and Lorraine regions were annexed back to Germany when France signed armistice with France in June 1940. Several examples of a German depicted on 10,000 Marks notes exist. Whether on purpose or not, the portrait appears as a ghoul when held on its side (90 degrees counterclockwise). The ghoul is seen as the French, sucking the blood from Germany by robbing Germany of these land regions. Even the "cap" and "tunic" look French in appearance. The notes are:
Pick 70Larger note.
Pick 71Larger note
Pick 72Smaller note. Thanks to Ron Wise for this image.
Here is the famous ghoul revealed when this note is viewed on it's side..

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The Czech word TEREZIN German word THERESIENSTADT Ghetto was established in early 1942 outside Prague as a model ghetto. It was not a sealed section of town, but rather an eighteenth-century Austrian garrison. It became a Jewish town, governed and guarded by the SS. When the deportations from central Europe to the extermination camps began in the spring of 1942, invalids; partners in a mixed marriage and their children, and prominent Jews with special connections were initially excluded. These were sent to the Terezin ghetto. Jews from the Protectorate, and later, by small numbers of prominent Jews from Holland and Denmark joined them. Its large barracks served as dormitories for communal living; they also contained offices, workshops, infirmaries, and communal kitchens. The Nazis used Terezin to deceive public opinion. They tolerated a lively cultural life of theatre, music, lectures, and art to display to officials of the International Red Cross. What the Germans didn't allow outsiders to know was Terezin had about 88,000 Jews deported to their deaths in the East. 17,000 remained by April 1945, when they were joined by 14,000 concentration camp prisoners evacuated from camps threatened by the advancing Allied armies.
On May 8, 1945, Terezin was liberated by the Red Army.
The notes are dated 1 Janner 1943. All the notes are signed by Ghetto elder Jakob Edelstein.
1 Kronen
2 Kronen
5 Kronen
10 Kronen
20 Kronen
50 Kronen
100 Kronen

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France Examples

The French, for their part, had an equal dislike of Germany during the World Wars. The following note is an example of how this dislike manifested itself. The French modified this particular note and used it as propaganda. Every time this bill was passed the message was clearly sent. The 20 Franc note originally has a picture of a Breton fisherman pulling in his catch with a rope. This was modified, very carefully, by inserting a picture of Hitler to make the fisherman look as though he is strangling him with the same rope. A portrait of Petain is also known to have been used, also less common than the following Hitler version. Another version, with Petain being nursed by "Mother France" on the reverse is shown in R.G Auckland's "Air Dropped Propaganda Currency". My thanks to Christina Smith (classydeal on Ebay) for this image and information.
Pick 100

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Burma Example A 1989 "democracy note" from Burma carries a more subtle message, but one that was just as challenging: 1 Kyat
"It's a one-kyat note from Burma which represents General Aung San. But what it doesn't represent is his daughter, (pro-democracy activist) Aung Sang Suu Kyi, who it was illegal to produce images of," says Gavin Grindon, lecturer in contemporary art and curating in the University of Essex’s School of Philosophy and Art History. He was previously visiting research fellow at the Victoria and Albert Museum, and taught at Manchester, Birkbeck and Goldsmiths universities.
"The artists who produced the note - when they did the watermark, softened the features of the General San, so that if you held it up to the light, you had a secret portrait of Aung San Suu Kyi."
We don't know the fate of the subversive artist - but what happened to the notes?
"They were withdrawn."
Gavin Grindon was talking to Radio 4's Front Row on 21 July. The interview is available to hear on the BBC website.