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Here is the Quick Grungy Poster tutorial that was originally written for Photoshop by Fabio at PSDTuts. I remade it for the Gimp. For this tutorial you will need some grunge brushes. Go to DeviantART.com and search "Gimp Grunge Brushes." You can also use grunge Photoshop brushes if you want due to Gimp supporting Photoshop brushes. If you want to learn how to install them, go here and look at my guide to installing anything in Gimp.