This is driving me up the wall. Two different computers (one at work, one at home), both are Windows 7 64-bit. Home version of Calibre is not 64 bit. Work is. Both are version 2.7. Both are creating 330 pixel high thumbnails. I can watch it create the thumbnails in system/thumbnails as I send the book. The portable version of Calibre 2.7 sends a larger thumbnail. So, I can use the portable version, it's no big deal, but it's just totally bugging me.