I have made a base and some recolors. After a while I made some improvements on the textures and just exchanged the old ones (just opened the object and built new DXT). The new textures show up fine in the game, but the preview pics (where you buy the stuff) didn't change. Is there a way to fix that?

Yep. Delete this file:
My Documents\EA Games\The Sims 2\Thumbnails\DesignModeThumbnails.package

This won't hurt your game. When you restart the game, it will recreate this file and make new thumbnails.

Thanks, that helped create new thumbnails for when you have clicked on the "first" thumbnail, the one that is next to all other objects. So I improvised following your info and deleted ObjectThumbnails.package as well, and that changed the "first" thumbnail Thank you!