GIMP Gate

The only way I've mixed is by being really obvious, like cartoon character in a photo background, so that it doesn't look like a failed attempt but an intentional combo (like Roger Rabbit and such). But then my style doesn't lend itself to photorealism in the first place. Though I think even with a very realistic style painting within a photo is hard to be seamless. I think if cohesion into one style is the goal, and one element is painted, it often might be easier to paint the whole thing based on the photo reference and have it all look like a painting.