This is to help you with GIMP:

Go to this website for fonts to download:
http://www.dafont.com/

To learn how to install font:
http://www.dafont.com/faq.php#mac

Go to this website for brushes to download:
http://browse.deviantart.com/resources/applications/gimpbrushes/
search for the kind of brushes you want, the brushes will be in packs together, once you chose one pack of brushes you want, click on it and it will bring you to the page for the brush pack you want, go to the right side of the page and click "download file"

To learn how to install brushes:
http://mygimptutorial.com/how-to-install-gimp-brushes

Go to GOOGLE to find textures that you want

To learn how to apply textures:
http://iffles.com/2009/10/26/adding-a-texture-in-gimp/

GIMP tutorials:
http://www.crunchyroll.com/group/Avii_Tutorials_%28and_others%29/forum?extra_param=531982&page=0

http://www.crunchyroll.com/group/Avii_Tutorials_%28and_others%29/forum?extra_param=532454&page=0

http://www.crunchyroll.com/group/Avii_Tutorials_%28and_others%29/forum?extra_param=581704&page=0

http://www.crunchyroll.com/group/Avii_Tutorials_%28and_others%29/forum?extra_param=531946&page=0

http://www.crunchyroll.com/group/Avii_Tutorials_%28and_others%29/forum?extra_param=453146&page=0