I suspect that this was done using the color selector. I found that the path tool works much better. At first I found paths much more slow and frustrating. But within a week the paths tool was faster and easier than using the color selection or magic wand to cut things out for transparent PNGs. It only takes a few day to get used to it. Excuse me if you already now this.
Uh, thatīs a combination of colour selector (the upper parts) and crazy cutting by pen. I liked the effect... I literally follow the outer line with my pen. Until Iīm around it... then I invert it and GIMP cuts out the rest. I wanted to get used to the pen and cutting out seemed to be good practice and there is a lack of motorbike stock here. (which I started with...)
Itīs not, that I donīt try to understand how to use pathes in general - Usually Iīm too lazy! Make a tutorial, hehehe