You can either import an image as a texture and paint it directly onto a surface, or you can import it as an image, get it into the correct position at the correct scale then explode it and project it onto a surface. You'll find both of these options over on the right hand side of the image import dialog box.

If you are hoping to do anything remotely sophisticated for IMVU...like map an avatar...forget it. SU's mapping tools are nowhere near up to the task. You'll have to use a 3rd party application like Ultimate Unwrap 3D. Such an application can be used in conjunction with SU by using some of the UV Tools ruby scripts, which will allow you to map in UU3D via a proxy obj file.