I am new too, i was having the same problem.

In order to paint anything in 3d tool, it is neccasary to first assign the texture to the object. In 3d paint tool there is a button that says assign/edit texture. You need to click it and assign a texture to the object using it. This creates a file solely of the texture for easier importing/exporting.

I recommended (others have really recommended to me) to use the .tiff format because its easy to export to photoshop where the real texturing results happen.