When using ‘Upload Applet’ to upload pictures to Gallery2, broken thumbnails may occur unpredictably. A maintenance will need to done to rebuild all thumbnails. The problem may be resolved by changing the priority of which graphic toolkit to use in the Administrator’s Site Admin page.

If NetPBM is the highest on the list, try lower it down below Gd. ImageMagick is not activated in this case. (Note: the hosting service provider is 1and1.)