They weren't being generated in 2.5, and I upgraded hoping to solve the problem. Same problem in 2.6.
When images are uploaded, they upload fine, however no Thumbnail is generated. When I try to insert the image, it inserts the full size image.
I have tried smaller image sizes to see if maybe my images were big. No help.
I have tried different types of images. No diff.
I have tried the fixes listed by WordPress, including the functions.php. No help.
I have searched these forums and see dozens of people having the same problem in 2.5 and 2.6 and being completely ignored. Is there no solution to this problem? This is a serious issue.