program that stitches together a bunch of image files into one giant poster?

[SMcNeill]

Your limit is going to be 4GB in size.   The reason is rather simple... 32,767 pixels x 32,767 pixels is the largest an image can be, and 32-bit images are 4 bytes per pixel.   32k * 32k * 4 = 4 GB.

Or...  An _UNSIGNED LONG in size.