Posting a product photo on an eBay auction listing or an online shop requires that you prepare it properly. Be sure to resize it for the Web and to edit the photo so it looks clear and vivid.
You can see this tutorial on resizing your photos for the Web. You want to be sure they are small enough that they don't slow users down, but big enough to reveal nice details.
You also want to be sure your photos look wonderful. The About Graphics Software guide features resources on Quick Photo Fixes.
If you complete all of these steps, you are sure to have compelling product photos that make your items more attractice and easier to sell.