www.iforce2d.net

This is available in Physics Body Editor, which is a tool similar to Rube2d with much smaller feature set. Physics Body Editor is open source, with Apache 2.0 license so you could even use it in your project.

https://code.google.com/p/box2d-editor/ ... erter.java

This text might also help in adding such feature:

http://astroboid.com/2010/06/mapping-bo ... tures.html

That is definitely something I would like to do sometime. The voting for it does not seem to be too high though...
I would probably try to do something like mentioned in this thread, where you could create a boundary for multiple images, as well as just a single image: http://www.cocos2d-iphone.org/forum/topic/112793
Note how the purple lines in the screenshot go around the edge of where multiple images have been overlaid.