My site is full of rounded corners on every box and picture, except for the thumbnails of user uploaded photos.
How can I use the Python Imaging Library to 'draw' white or transparent rounded corners onto each thumbnail?
Might it not be a better idea (assuming HTML is the output) to use HTML and CSS  to put some rounded borders on those pictures?  That way, if you want to change the look of your site, you don't have to do any image reprocessing, and you don't have to do any image processing in the first place.
Draw rounded corners on photo with PIL