A strategy: For each image file you can have a text file in the same folder describing the image.
The text file for zzzz.jpg (.gif, etc.) should be named zzzz.txt or zzzz.htm (or .html), the HTML versions supporting formatted text, links, etc.
It should be possible to print out an image and its descriptive text on a single sheet of paper.
Each image folder should have several files which announce this strategy: for example, readme.txt, readme.htm, and index.htm.
This strategy can also be used for other media, like sound clips, video clips.