As you can see at http://www.micheledelbecaro.com/bn/ the site don't display the thumb of images, slideshow works correctly and the thumb of the album are visible, any ideas? i've already tried to precaching image but the 300x300 thumbs are black images

Did you clear the image cache before changing the sizes? What are the values used?

yes i clear the cache, the value for the thumb is 300

Any cropping settings? Those do not use px values but %!

Crop thumbnails setting was the problem i disabled it and now all work!