The menus are more than just graphic images, I take it.
Yeah, the menus are split into hundreds of tiny tiles (8 x 8 pixels I believe), and each pixel can either be a 16-colour bitmap texture or a single colour fill. There's a maximum of something like 16 palettes for the whole screen, so most tiles share palettes with others.
That means that to edit a menu background, you need to understand which tiles go where, which palettes are used where, and what will be affected with each palette change. PD's art team presumably had some strict disciplines and smart tooling to build them in the first place.
There are also two layers of tiles in different files, one for the background images and one for language-specific elements. On top of that, there's a system to place common icons such as Home, Race etc. from the top bar, to define which parts are clickable and what they do, and to place dynamic widgets like event license and power restrictions, prize money, best result, total money, current car, a render of a car (e.g. the dealerships or garage), and probably more.
In short, the GT mode menus are very, very complex to edit! Merging the Vauxhall dealer icon onto the North City background took a full week of work with a hex editor, and I wasn't even making anything new there.