Well, I would personally use a tilemap and assume that all tiles are solid squares, however if you're dead set on pixel-perfect image collides (if you're doing summat like Thrust this would be advisable) then I'd use a tilemap which tells you what the image is in each tile position and then check for collides with all the nearby ones. Checking every tile in the level via a list of types would be the maddest idea in the history of everything and something you definitely want to avoid. Very SLOOOOOOW.