Make a solid object curved based on transparent pixels

A student of mine is working on a prototype of a basic platform game. There's a platform which is built as a "tiled background" object with the "solid" behavior. That object contains an image of rounded hills. Is there a way to make it behave such that a sprite can walk over the curve of the hills (the rest of the image is transparent)? At the moment, it is just behaving as a solid rectangle based on its containing box.

Not with a tiled background. If you want a collision shape other than the object's bounding box, you'll have to use a Sprite for the terrain and set its collision polygon accordingly. Information on collision polygons on this page.

Thanks. That worked like a charm.