[Jenny]

Many thanks for the link!
Do you claim some special copy rights for this bench-photo, or am I free to use it in a (non?)-commercial way?

Shadows always are very difficult, I know! Photoshop offers a filter called "Distortion Map...".
Although it's not that easy to handle, it'll reveal quite realistic results.
Google will surely find a lot "phpotoshop tutorial distortion map", and at least one of them will work out fine for your intention!