Open the background, press Ctrl-A (select all), Ctrl-C (copy), Ctrl-W (close). If necessary, make your layout window active, then paste the background as new layer (Ctrl-V).

In the layer palette, click and drag it beneath the other layer(s).

I've got my transparent layout behind all my text,

I want to add a background lets say something like a wooden plank or a sort of gradient background to go behind my words.

So how can I fill in all the transparent parts with the photo that I want?