The Gallery 2 import is good to transfer all photos from old, Gallery 2, to Gallery 3. But sometimes the thumbnail is not converted completed. Some photos thumbnail are missing.
So, I installed Rebuild Items module.
Gives the administrator the ability to rebuild items (thumbs and re-sizes) as well as EXIF data. It marks the items in the database as ‘dirty’ and Gallery will then attempt to fix those items that are marked as dirty.
It can rebuild thumbs and resizes in specific album.
1) Download the module from here
2) Decompress the zip file, and upload to the module directory of your G3 install.
3) Activate the module: Login -> Admin -> Modules. Check the box next to the module. Click update at the bottom of the page.
4) Click the album you want to rebuild, select the menu: Albums Options -> Rebuild Items
5) It will give you the options to rebuild Thumb and/or Resize
It is good and simple to rebuild items for Gallery 3.