If you want to load a bitmap into a texture and display that on the screen, well, that's substantially easier. There are plenty of good opengl tuts out there which show "drawing your first texture"

To get a subsection of a texture to draw, you need to alter the texture coordinates of each vertex of the rectangle drawn to the screen.

To draw from one texture to another, you will first have to draw the first texture to the screen, then use the method described above to copy from the screen (or should I say framebuffer) to the second texture.