How to make a beautiful and at the same time unobtrusive background?

Question

Design

In the window with the game there is a main picture, which is replaced by one another every time.
In the previous version of the application: there were six identical backgrounds of different colors (purple, gray, red, yellow, blue and green). Some of the pictures did not look good against the background of one color, but fit perfectly with another (for example, red on red is game, but on yellow or gray it’s already good). And I had to change the background for each specific picture.
An update is being prepared, and now I want to make sure that any picture is combined with any background color.
What can help? Blur / darken / brighten the background? Stroke / Outer Glow of an image? Or can you come up with something more interesting?