1. We're going to paint graffiti on a wall using photoshop. This is a pretty simple tutorial that shows how to make a wall image overlay in short. So go ahead and find yourself a picture of a wall, you can also use mine!
2. First you will have to select an image that you want to use. I just created a rectangle box with some text on it... Of course you should find or make an image yourself! In the first screenshot you can see the picture I used. In the second screenshot you can see I pasted it onto the background picture of my choice (and also made it bigger). It does not have to be a wall. It could be a car, a computer, on a person to make the appearance of a tattoo, or any other picture that you'd like to use.
3. Now we're going to adjust this image to the direction of the background. To do this we're going to free transform the new layer (edit - transform - perspective). It's not an easy tool to use, but when you play around with it you'll be able to achieve the result as shown below! Move your picture around the background making sure that the scale and perspective match the area you're placing your picture!
4. Now in this last step just change the blending mode to overlay! And there you go, graffiti on a wall! Like I said it's a simple tutorial and it can be used for almost anything! Have fun with and play around with it a little bit...