Drag and drop is the way to go. However, SU doesn't like being partly minimised in order to do that...it tends to send the toolbars all over the place.
If you want to avoid that happening, simply import the image file...but not as a texture; import it as an image then explode it. Whenever you choose an image format for import you get a set of radio buttons on the right of the dialog box which enable you to choose the manner of import. In fact I hardly ever import an image as a texture, even when that is how I intend to use it.
