One of the biggest directories I have in my web site concerns Physical Science lessons. Thus, I have a separate images subdirectory for those files. On the other hand, many of my directories or folders only have a few images. In those, I simply include them along with the HTML files. If there gets to be a sufficient number of images, I will also create a separate sub-directory for them at that time.
Some people give descriptive names their images.
I used to do that, but now I name the image files according to the HTML file they relate to. Thus, if the filename was FILE.HTM , its images would be FILE1.GIF, FILE2.GIF , and so on. In this way, I can keep track of what images belong to what file. The School for Champions logo is one color for all lessons in one subject matter. That image file is located in the class directory or its image sub-directory.