Well, say you wanted to replace the icon for the Ranger - then you'd make the image the same size as the Ranger cameo, then save it as a tga file under the same name. You could then just drop it into a folder in your generals directory called Data/Art
Simple - you'll need to find the relevant data for the image you want to replace with FinalBig by rooting thru the appropriate big file.
GenDev has Gen Cameo Mapper, so you can generate the necessary mapped image files and everything. You can also put more than one image into a file, like the current UserInterface files, and map them all together.