Usually that type of thing is better done in Photoshop.
But to do something like that In Illustrator, you could use the rectangle tool to draw a box, then apply a Brush to the stroke, from the Brush Palette.
There are a bunch of brush libraries in the drop-down menu.
You would probably also have to Object/Expand to match the color.
Or, you could Live Trace an actual photo that has the rough edge that you are looking for, or perhaps a scan of your own brush strokes.
Just a few ideas.
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