At the bottom of the "UV Image Editor" window, open the "UVs" menu and click "Export UV Layout."
You will then get a "Save as" menu where you can specify where to save a PNG file. You can also change it to a different resolution or image type with the options in the lower left.
The default exported PNG image will have a grey outline of the UV map filled in with transparent white. The rest of the image is also transparent so you can put it over top another image in a photo editor like GIMP and still see the image underneath. Good luck.