The easiest way to learn how the image ini files and the image files work together is to look at the ones that come with the game. The image ini files are in INI.BIG / INIZH.BIG in:
Data\INI\MappedImages\TextureSize_512\
And the image files are in English.Big in ZH and Textures.Big in Generals.
Open one ini file and open one of the TGA files and compare them. You will see immediately how it works.
Just note that the EA games ini files are all wrong. They add one pixel to all image coordinates. Instead of following them, just use the exact pixel coordinates of the image edges that you get from your graphics editing program (paint shop pro, photoshop, whatever).
Just put your mouse cursor on the top left pixel (zoom in the view to be accuarte) and write down the coordinates your graphics software tells you for the top: and left: coords, and then put your mouse cursor on the bottom right pixel and write down the bottom: and right: coords. Do not add 1 to them like EA games does. This results in a black strip or a 1 pixel strip of another image appearing at the edge of the cameos in the game.
If you need good free graphic editor that gives you X Y coordinates of your mouse cursor, you can download PhotoFiltre:
http://www.photofiltre.com
It is small and very good.
Also note that EA games use different image sizes for both the large portraits and the smaller button images, and are not consistent. To get the best size, take a screen cap of the actual in-game user interface and measure the size of the cameo images as they are displayed in the game - that is the size you should use.