How to create/imitate a crack in a picture in Photoshop
How can I add crack to a picture in Photoshop cs5 (with or without using custom brushes) like the one below?
I tried with brushes, but it didn't look realistic.
Step by step, assuming you have used Photoshop:
Create a new layer, and draw an irregular line using the brush tool.
Zoom in and play with the line's opacity (you can delete or opaque certain parts), apply some smudge and also some shadows using blending styles.
Alternative, you can use a crack brush such as this one:
The 'making it realistic' part is up to your Phothosop skills. Unfortunately, there's nothing we can do to explain this part. It comes with practice.