The only small and light
program that comes to mind is AutoStitch
, the forerunner of AutoPano and a couple of others.
But I just tried it and it distorts stitched screengrabs, I think this because the amount of overlap and the size of the individual grabs varied and the software also assumes that the individual images were made by a camera and therefore tries to compensate for lens distortion.
Otherwise, try hugin
, it's not small and light, (can be run portable, IIRC, just download the 7zip version and extract), but you can edit the 'camera lens' attributes so as to remove most, (if not all), distortion plus it has a lot of other settings to change perspective. (I'll have a play when I'm fully awake.)
Also, is this a specific map site ?
There are programs/scripts written for some map sites that automatically download the individual map 'tiles' and then create the bigger picture.
EDIT: I don't think hugin is going to fit your needs, far too fiddly to combine just a few grabs.