When you take a photo with a digital camera, it is given a file name (eg on mine, images are prefaced with SD then a number, in ascending order, so first saved image is SD001, then SD002 etc). When you upload to the computer, you can change the file name to pretty much whatever you want. 

I've always done that so that they're a lot easier to store / find, anyway (that way, I can use a quick keyword search in my image folders, which have several hundred each).  I might have 5 or so images of one item, but instead of 5 file names of just numbers, I group-change the name (by selecting all the images, then right clicking and selecting rename) and would put brand, colour and type - by selecting all images and renaming at once, they're automatically assigned a number to differentiate then, so I'll end up with image titles like brand colour type (1) , brand colour type (2) etc

I don't know how much that does help eBay rankings, but it definitely improves google image search rankings.