The hugin panorama creator is a great tool for photographers who use Linux. It has great abilities to merge photos into a nice panorama image. Hugin, The Panorama photo stitcher is absolutely great. I just tried it with two photos I took some time ago and jumped on my terrace to make a series of 6 I could merge to a single image. It worked outstandingly.
This shows how you can connect the different images with points where the same object or same pattern appears. (I know these images are horrible, I should have taken 2-3 more of that building. Btw, this is the town hall of Flensburg.
The hugin website offers a lot of tutorials which I definetly going to have a look at.Puzzling images together in GIMP can really consume a lot of time. *sigh*
Here my first results: