I am currently using ZenCart to build my store. I've noticed that some images look blurry while others are not. In fact, they come in and out of focus. No rhyme or reason to when this happens either. I have optimized all the images in Fireworks, I believe all the same way. Can anyone tell me how to fix this problem? Could it be a scripting issue? Help!

My site is
www.tot2teenonline.com. An easy picture to look at is the one for baby bedding. That one doesn't seem to ever change. Strangely, when I uploaded it a couple of months ago, it looked fine. But for the last month, it's been blurry. Seems like more and more pictures are doing the same thing. Please ignore the logo, this is in the works. It's the category icons, and some product pictures that I am talking about.
Thanks for your help!