That's pretty good!
Do you use Photoshop? I work with Photoshop full time, professionally, so if you don't mind, here's some advice on how to make these pictures even more realistic (please don't take this as criticism, just trying to help):
- If you go to Image > Adjustments > Curves, you can adjust the color of your face. I'd put a little less blue and red and a little more green in there.
- Be a little more precise with scaling. On the last photo for instance, your head is considerably bigger than John's.
- I'd use a small eraser tool to go around the edges of your hair. The edges are a bit too 'hard' now, it makes you look almost like a cardboard cut out.
- Especially on the picture with Paul, I'd make your face a little sharper or Paul's a little more blurry with Filter > Sharpen > Sharpen (or Sharp Mask).
- A trick I often use is Filter > Render > Lighting Effects. This way you can put a spotlight on a photo. If you combine two different photos and put the same artificial spotlight on both, it'll look more like these two parts actually belong together.