How do I paint in UV edit mode?

Does anyone know how to paint in UV edit mode, because I dont. Also, does anyone know how to put textures on a 2D object and leave the rest invisible? Like making some textured and some see through?

I appreciate the help, thanks.

Here is one place to start:

http://download.blender.org/documentation/htmlI/ch11s05.html#materials_uv_textures_painting