Many times you see other peoples auctions with your images on them !
A very good way to prevent/deter this is to add a transparent watermark to your images, adding your user ID is very easy. It can be done on line by using an eBay auction image host that has this free function.
You simply enter the text for your watermark to create it, select the option to add at upload, and the watermark is automatically added to the image.
Below is an example for you, and you can easily change the font style, size and postition or your watermark.
(eBay only allows small images on guides, sorry)