Is it important that the physics are derived from a single .png? I think you're better off using
Tiled and SetSpriteShapePolygon() to process Object shapes. Phaelex just updated his
importer too. The one I use is based off this
thread.
Load in a tmx map, assign a sprite per Object and modify that sprites polygons with something like this (using commands from the old version)
Just remember to keep your objects under 12 points, convex, and build them with the points going in a clockwise direction.
iPointCount = tmx_GetObjectPointCount(layer, object)
for k = 0 to iPointCount -1
iX = tmx_GetObjectPointX(layer, object, k)
iY = tmx_GetObjectPointY(layer, object, k)
SetSpriteShapePolygon( spr, iPointCount, k, iX, iY )
next k