Hi,
Use object blend modes…
Place a texture image on top of the image you want to make distressed (It could be an image with cracks. We have some like that in the library). Make sure the texture image object is selected and the “Object” tool icon on the left panel is selected. Then press “P2″ tab in the “Current object” panel. Give it the blending mode “Multiply”.
That's it!
You can experiment with other blending modes and also adjust the “Opacity” in “P1″ tab below 100% to see different results.
I strongly recomend you learn how to use blending modes to design your graphics. It is very useful for various purposes…
For example, if you want to cut out a text from the image below. Add a text on top of your design, then give the text the blending mode “Erase”… Try also to set the text bleding mode to “Alpha”… This will make your design only be visible inside the text.