I find exactly the same problem with the size of the thumbnails. They are too small and no way to change them.
I want to upload images at about 640×480 and have the thumbnails at 200x? ish (if you can call that a thumbnail…!)
Thing is, I had a look through the WP source code and the thumbnail size is hardcoded with no way to use a plugin to change it !!!
Someone correct me if I am wrong
