[Mooff]

It's a browser cache issue. We had that in the sprite addon aswell. For some strange reason the modern internet browsers don't get when the filename stays the same but the contents of a file have changed. Meaning it will show you an old slide1.jpg out of your browser cache which doesn't match the slide1.jpg on the server. Hit force refresh (ctrl+r or ctrl+f5 on firefox) and it should be solved - sidenote, neither opera nor chrome have a force refresh anymore afaik, there the cache has to be cleared manually.

Osbes did fix it in version 0.8.0 of the sprite addon (which is currently in alpha stage) by using different file names each time he creates a sprite. As soon as he releases 0.8.0 i'll snatch that code snippet and update the slider.