I'm not a photographer but a graphic designer.
In GIMP, I only discovered two methods of improving the quality of an photo:
Filters > Enhance > Unsharp Mask Noise > HSV Noise
Basically, I would like to remove some blurriness and 'fake' high resolution, if possible (I don't mind adding some noise).
I like this final result: http://www.blog.spoongraphics.co.uk/about (the guy in the picture).
Any other suggestions?