The simplest way to hide additional information in an image is to simply write it on it in a clearly visible way. If the image has a very high resolution and is therefore displayed at a very small size on the screen, such labelling can be almost invisible, depending on the background. Very large images should therefore always be examined very closely, i.e. opened in an image viewer and enlarged.
My favourite image viewer for a digital eternity has been the freeware IrfanView. Also recommended from my little world view is xnview. Apart from these two, however, there is a plethora of other useful tools that serve well as image viewers.
A thoroughly classic, visual method of hiding information in images, which has been used long before geocaching games, is to place small details in an image which, if you know the code, can be deciphered.
An example is this image with the blades of grass by the stream.
The short and long blades of grass are Morse code and result in the text, “Compliments of CSPA MA to our chief Col. Harold R.Shaw on his visit to San Antonio May 11th 1945.”
A salute to the head of the US Censorship Board at the time, who went to great lengths to examine the mail for anomalies. Some nice stories have come down to us, such as the fact that all the hands in a consignment of clocks had been adjusted because they feared that a code was hidden in the preset time. In another letter, they found a knitting pattern and only delivered it after an employee had reknitted it and thus proved that it really made a jumper. None other than Charles Dickens used this very form of encryption in one of his novels, “A Tale of Two Cities”, and thus presumably also invented it. Conversely, jumpers were sent by German agents in England during the Second World War which, when unbuttoned by the secret service, yielded threads in which knots attached at certain intervals contained a code.
Point ciffres are historically documented and were used 400 years ago. Here, a cross of axes is used, one axis being the letters and one being the order of these letters in the text. For example, stars as in Blaise de Vigenere’s book “Traicte des Chiffres, ou Secretes Manires d Escrire” or bees as in the French magazine Spirou can then be used as dots and linked to form an image with sufficient camouflage.
Much can be learned from the intelligence methods of the time and the two world wars and the Cold War, when computer-related encryption was still unknown and information had to be well concealed and transmitted in encrypted form. Thus, in intelligence terms, it is classic to mark certain letters or words of a text inconspicuously. With an almost invisible dot over certain characters or a slipped typewriter letter, an almost random smearing on the paper. Something that distinguishes parts of the text from others. Hiding information in containers of some kind (for example, images or text) is called steganography. A subspecies of this, hiding in details of these texts and images has the nice name semagram.
Once you have found what is encoded, or if the picture clearly indicates where it contains the information needed to determine the coordinates, you “only” have to decode it. The easiest way to do this is to first compare the best-known techniques and code lists. How many different code characters are there? Two? Then maybe dual numbers, Morse, Braille, (see binary codes). Or do you need to research something specific and use that? If so, hopefully the listing, title or hint will give a little nudge in the right direction.
If you haven’t found anything, you can try the following post, Technical Image Analysis