Just as countries use counterfeiting against an opponent, so do they use "hidden" images to do the same. Several image examples are in the lower section on this page.

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 Terezin Ghetto notes were made to segregate money flow in the ghetto, but also to symbolize the "fair" treatment all Jews were receiving at all ghettos.

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.

5 chiao notes (50 Cents)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. These notes are:
Pick 485
Pick 486
Pick 487
Here's an example.

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 J63, 10 Yuan, 1938

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

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 Alscace and Lorraine areas annexed to France from Germany. It is no coincidence that the Alscace 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..

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 istself. The French modified this particular note and used it as propaganda. Everytime 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