Gaming backdrop question

[Iarolonja]

I made a game years ago in blitz basic and am currently rewriting it in C++. Its a simple maze type game a bit like chips challenge. Currently I have the screen divided into the gaming window in the top left part of the screen, which currently has a fixed resolution, and the rest of the screen is taken up by just a black border. If the resolution is changed then the amount of black area will change.
I wanted to know what is the best way of filling this space to make it look a bit more pretty. I could draw something in a Paint type program and just load it in but then I would imagine I would need a fixed resolution otherwise it would look off when it got changed. Another possibility is to make a simple tile type image and just repeat it to fill what ever space there is.

Is there another option I may have missed?
I'm using allegro 5 as my programming library and am entirely self taught at programming so may have missed something fairly obvious here.