Replace "your.username" with your actual user name.
The picture is clickable and the link takes you directly to the picture hosted on Picasa Web Albums.
Due to a Bug in Google App Engine, contents of large albums (>~300 pictures) cannot be displayed properly at the moment.
Thanks to partial responses the script now also supports showing albums with 400 or more photos.
I finally uploaded the project to Google code. Check it out at https://github.com/pmoor/rndpic. In addition, I added the ability to specify custom sizes for the thumbnails as well as the option to turn off the link generation.