I've used this: http://www.casa.ucl.ac.uk/software/g...magecutter.asp

I usually make my 30000x15000 pixel image in PS. Somewhere I've posted the maximum file sizes I was able to get working and the command line function I used to maximize memory used to do so. If you search for 'gmap image cutter' on here, you should find the info (it was a fair while back). There are others out there now that are more advanced, though, I'm sure. This was/is simple and worked well at the time.