Wow, this came out a lot better then I expected. The trick is to use brightness and contrast just enough to make his face lighter, without making it look totally washed out. Mines a bit washed out, but I'm sure with a longer bit of twiddling someone could make it match more perfectly. It takes more tweaking then I spent on it.
What I think really tries it to the photo to the image is that I drew on his face to try and match it with the actual painting. I color selected the lightest color on his neck and drew some highlights on his face. Be sure to Gaussian blur it too, It's important because it smoothes out the face a bit. (Upping the contract makes it look ugly) And then I did the same for a few dark spots. It isnt a perfect match, but it's a hell of a lot better then it was before.
I also used the liqify tool just a *tad* bit to make the beard look more like it was part of his chin.
Here is my
photoshop file so you can see exactly what I did. That way you can try it yourself and make your version a bit better. Good luck. Remember, to stop sucking at photoshop all you have to do it keep working on stuff. Even I'm not too great when it comes to realistic photo-manipulation (I'm more comfortable at abstract) but when you do lots of little things like this Jesus, you can't help but get better over time.