There are projects like sorl-thumbnail which is a great and easy to use django app for thumbnailing. But it also depends on PIL so you can't use it, that is bad news. Good news is since in jython you have access to all goods of java libs you can possibly use one of java code snippets available on the net, google: "java how to create thumbnails" (I didn't check any of them).
On the other side, in my project, I used ImageMagick's convert command-line tool (-resize or -thumbnail option). It gives very good quality thumbnails which are also optimized in terms of size if you use -strip with -resize or -thumnbail option alone (unnecessary info is stripped). The downside is that you need to operate on files and then fetch thumnbail back to mysql.
greetings,