Adding Gameplay Screenshots

[Cookz718]

So im currently trying to put a Theme together inspired by Modernist. But im having some issues in trying to include Screenshots from Gameplay.. Also i want to put a gradient background or like a fixed background that i made from photoshop to be in the background that way the background doesn't constantly change every time i click on to a different game. (Yes i am a total newb at this). Would be nice if someone can explain to me what can and how can i do plz

(Note: The 2nd Image with the Orange background is how i currently have it at the moment)