Flask is a microframework for Python based on Werkzeug, Jinja 2 and good intentions. And before you ask: It's BSD licensed!
Wow! Creating fake-o images as wireframe placeholders for prototyping has never been so easy! Simply add some regular-ol' HTML image tags, and lowersrc will spring to work – dynamically creating images (based on your customisations) – leaving you to get on with your layout.