you need to have an alpha channel to allow photoshop to see where the flowers should show through and where they shouldnt. It's like a transparency channel. And for that you have to save as tiffs or tgas (never save as bmps!)

What I would do is think of it in layers, front middle and back. Have anything behind the house as the back layer, the house as the mid layer. When you render the flowers it's probably worth assigning a matte black object to the house so that it mattes out anything it's covering (go to matte opacity in the shader and change it to black hole and render the flowers again and you will see what I mean).

Then comp them together in photoshop as normal.

Alan