What you need to do is create an animated GIF. GIFs are currently the only image filetypes that contain animation (you can also do animation in Flash but I don't know if eBay would permit you to embed a Flash file on one of their pages).
If you have Photoshop CS3, you can create animations within the application (previous versions came with a sister application called ImageReady specifically for making animations and other web graphics). Here is a link with lots of animation tutorials:
Photoshop Animation - Tutorial
One thing to be aware of, however: GIFs only support a maximum of 256 colours, so if your original image is a photograph containing subtle gradients and colour-shifts, it's going to look a bit crappy when you convert it to a GIF. As a fairly frequent eBay customer, I have to say that the thing that attracts me most to an item (and a seller) is if they have included good clear photographs - I'm afraid cheesy animated effects in a sub-par image would not impress me at all.
Just sayin'.