I don't think I initially thought Dan Radcliffe looked like my version of Harry, but he grew on me. ;D Now, I automatically visualize him when I'm reading fanfic. Snape has never looked like Alan Rickman to me. I always think of him as younger, taller, thinner, and uglier. Though I always hear Alan's voice and speech patterns.
As for art recs, well, here's just a small handful. There are literally hundreds more pieces I absolutely adore: