A decent computer really helps with this

Just been trying to stitch together 13 pics and well enough so that you can't tell, the pics were taken without a tripod so it's a nightmare, have to rotate them as well as cut bit's out. I'm doing it a different way this time, cutting out the rider, pasting him on as a new layer and using layer masks to blend the edges into the background, seems to work quite well apart from the picture now being about 5000 x 4000 and doing it on a cheap computer :'(