How to paint a texture on, based on the way it was mapped on with UV? I UV Projected an object, but now I want to paint certain parts of that map onto it without the whole thing. How can I do this so that the parts I paint still look like the same parts of the map?

EDIT

All the settings you need to see are in this screenshot :-)
In texture paint mode you can either 


Paint and modify the existing texture image such as (.png) file.
Paint on a new texture and layer the textures so that one covers the others.
Paint with a brush from one image to another final image ... rather like a clone tool from GIMP


A video tutorial for texture painting is probably a necessary resource for ease of explanation and understanding.