GIMP is an awesome open source image editor but one of the things that people found really annoying about GIMP in the past was that it doesn’t come in a single window. It has 2 or 3 separate windows which makes it hard for people to move it around and in my opinion it doesn’t look very professional and streamlined but thankfully that is going to change because the newest version GIMP, 2.7 and which is currently in beta has a “single window mode” which allows you to put GIMP into one single window.The “single window mode” option will be available under the “windows” menu.

bit more here