Hello IeMS, Welcome to Talkgraphics!

If the photo you want to knock out of the background is not too complicated you can use the shape tool and trace around it to create a shape over top of the portion you want to keep, select both the photo and the shape and hit ctrl+3 on your keyboard to intersect. To give it a shadow you can either use the shadow tool and use the shadow controls on the infobar to adjust your shadow blur, transparency etc. or when you make the shape to knock out the shadow copy it to the clipboard and paste that copy in after intersecting, offset it, and send it to the back. What this will do is give you a vector shape that you can manipulate with the shape tool, stretch, skew, apply transparencies to etc in different ways that you can't do with the shape tool.

You can ask as many questions as you like