How to make a seamless texture in GIMP, easily and quickly?
In five self-explanatory steps, see below:
For the final touch you can use some manipulation of brightness/contrast to somewhat remove both blur on the original photo and the blur effect applied during the "make seamless" process, and you have it done.
PS The real texture is 2048x2048, only 512x512 was used for the purpose of this tutorial. ;-)