I usually use the game to guide the visual design. I start with the worst programmer art imaginable (circles, stretched triangles, capsules at weird angles) and upgrade only once it becomes clear the existing art won't cut it. And above all, I try to make things readable: make sure the player can see and differentiate themselves, hazards, and goals/powerups as easily as possible. But if there's one thing I've learned from my artist friends, it's that the final product never looks exactly like what's in your head - part of the artistic process is learning how to incorporate quirks and limitations rather than fight them (easier said than done). The game art you show looks excellent though!