Not sure if this is a actual bug as much as proof that the collision polygon auto-generation feature on sprite image import is not done yet, but importing this picture:
<img src="http://www.thelure.se/C2/Bugs/flag1.png" border="0" />
results in a complex polygon like this:
<img src="http://www.thelure.se/C2/Bugs/flag1result.png" border="0" />
Are complex polygons ok with the collision system?