IMHO, I would make one seperate tutorial about UIFILEs, because they are the most important resources in MSN Messenger.
Editing PNGs and other images is a lot easier to do. But UIFILEs are a lot more complex. They define the way thinks are placed and what they do. (calling what function) UIFILEs also have got some confusing declarations (like padding/borderthickness/...) which new skinners can scare because they don't know what they do and how they must edit them.
Use enough code examples, and this tutorial will be a great base for new skinners!