There are a lot of great images on stock photography websites these days. They are no longer just the kitschy, stale image selections they once were. This can sometimes mean spending too much time looking through hundreds, if not thousands of images! Other times, no matter how hard I look, I know my ideas are just too specific to find the perfect image. I've learned to pick a few that stand out to me and envision the potential to take them where I want to go, as it is with this photo. This is when I get to play around in photoshop. (These images will then become backgrounds for postcards, brochures, etc).
I find the key to combining images is all in the lighting (intensity, direction and shadows). In the photo to the left, the flowers may feel a little bright, but in this case they were meant add a playfulness and so I didn't mind. With an artistic eye, I don't think there is an absolute moment that two photos perfectly fuse together. I don't want to get caught up in fine-tuning it to death. Nature itself isn't concerned with composition and yet it's very successful at visually communicating.
This combination of photos is not about convincing the viewer that the fish are at that location, so the difference in lighting wasn't an issue. The image instead is more about creatively conveying a concept. The fun part is how one image can convey several ideas. In this case the image was used in a message series titled INSPIRED.
Mostly, as I look through the images I've worked on, it's the creative and fun aspects, much more than the believability, that I enjoy most. Here is an image that I got to add a VW bus and make look like an old photo.