A good background image can really make your Bio 'pop' and show your personality.
We really, really like Unsplash, who offer high resolution, quality and beautiful images royalty free. Check them out here: https://unsplash.com/
Of course, feel free to use your own photos or images.
Always remember to check out what they look like on the PC version smartphone, remembering you don't want fuzzy or pixelated images.