I have been making changes to my site and backing up my work. The background images that are used in the CSS have stopped displaying correctly in most browsers. What could cause this?

I thought it was a new sprite that I had created. I had changed the css, and find out it had errors, so I replaced it with the old CSS to see if that would fix it. No it didn't fix it. The images still weren't showing, so it seems that it isn't the CSS file, but something is causing the css file to not display correctly. I have been backtracking my work. The funny thing is that it does o.k in some browsers now and yesterday it was all fine. It just suddenly appeared overnight.

If it worked yesterday I would think that it must have been something I did then. But I don't think so.

Where do I find what is causing this? Site is: esl in sider (dot) com