It took me a lot longer that I estimated, but I have everything in place now so hopefully I can start focusing on level design.
It is divided in several "screens". The Image Editor lets you load files, set their UV coords, set origin, and if they are animated sprites you can attach them animations.
After an image is prepared, you can load them on the back/fore ground, or use them in the Part Editor to create a box2d part:
The Part Editor asigns physical parts to an image. Basically 1 image = 1 object. You can also add shapes by using circles, rectangles and polygons; each with their own physical properties.
Then come the Object Editor which puts together the parts to create links/trees of physical parts joined by revolute/prismatic joints.
And there's also de physics editor which, after loading a background image, let's you draw the global static physics of the level for the character to collide with:
Character positioner (lets you decide where is there character going to start and in the initial position).
Sensor Editor (lets you put sensor all around the level that trigger events)
Victory editor (lets you choose the victory conditions for the level)
I'm planning to include this editor with the game, but probably I need to make it easier to use and hide a lot of details from the user. But that's for another time...
I need a character editor now...