The possibility to select into a GuiImage only a part of the image (a rectangle) would be rather useful. The use case I have in mind are "image atlases" with icons, buttons, etc, compacting in a single file a collection of images to be shown selectively.
Of course, the final visible effect can already be reached with several separate files, one for each image.
However, particularly with GUI elements, which tend to be many and small, the overload in file size (as the file header is significant in rapport to the total file length) and above all in connection time (each image requires to establish a server=>client connection) could be significant.
Especially for very small icons (16x16, 32x32 etc) it's indeed a good idea to use sometimes like an atlas texture.
