PS3Files
PS3files.net is a great resources for finding 100's of pre-built
custom themes. Not to mention you can directly upload your
theme to PS3Files directly from the PS3 Theme Builder
application.
EyeCons
This site is a must if you are looking some stock icons to work
with. Chris AKA TheMethod has created several a base set
of Vector icons that are available for anyone to use in the
creation of great PS3 themes.
Paint.NET is free image and photo editing software for computers that run Windows. It features an intuitive and innovative user interface with support for layers, unlimited undo, special effects, and a wide variety of useful and powerful
tools. It's the perfect solution for creating / editing
your icons.