This might sound like a lot of extra work, but in some cases it might be worth it. I once created a script with one of the popular image modules (I think it was Image::Magick but I'm not 100% positive). This script had one simple objective: grab all the images off of my digital camera (which attached to the operating system as a regular drive) and create thumbnails for them. I was extremely disappointed to see the filesize of the thumbnails generated by the module. I compared it to the filesize of a thumbnail created by using Paint Shop Pro 8 and the difference was incredible!

So if this is for something where the thumbnails are only for images that you put up yourself, it might be worthwhile creating your own thumbnails. Of course, this is probably out of the question if you are generating thumbnails of images uploaded by users. But just a warning.