Many of the UI images in UI.package are in a custom EA format - to get technical, they're in pairs of ATI2-compressed DDS files using YCoCg color space. This tool will take a pair of these images and convert them into one bitmap (.bmp) image which any image editor can modify. Then the tool will take a bitmap and convert it to the EA custom format, producing a pair of files which can be packaged and used in the game.
The UI image pairs have the same Instance ID, one has Group 0x00064DC9 and the other has Group 0x00064DCA. Be sure to load each in the right place in the tool or you'll get very strange-looking results.
Bitmaps must have dimensions that are a power of 4 to compress correctly: 32, 64, 128, 256, 512, 1024, 2048, etc.
