Combining large image file can be very tricky. After hours of trying and googling, I stumble upon a cool image processing tools named netpbm. It solved my problem immediately.
I have a very large tiff map image files that need to be stitched into 1 large map. Common image processing software such as Photoshop and Gimp will hang on my laptop since the total file size is over 5 GB.
With netpbm, combining those images is a breeze.
1. Download netpbm from http://gnuwin32.sourceforge.net/packages/netpbm.htm
2. Do not forget to download the "dependencies", you'll need it later on
3. Run tifftopnm to convert all tiff files to portable bitmaps format
4. Run pnmcat -lr or -tb to combine the images
5. Run pnmtojpg to produce the end result in jpeg format