I have been noticing my friends with online stores have been having problems with resizing their pictures to the quality they need to sell their products. With my stores on etsy and ebay and of course my website and blog I use something called Pixresizer developed by Bluefive software .
Pixresizer is a free download directly from the Bluefive website. I find this to be the easiest way for me to get my pictures to the size I need and the quality I want. You can use the pre set sizes or custom size your pictures to what you need. You can upload and resize multiple pictures at a time or do them one at a time. Your original photos are not altered at all and you can save the new images in a different folder. I organize mine by size, that way I always know what pictures can be used where.
Try it once and you'll be hooked. I was, and pixresizer is all I use.