Oh yeah, if you use the image sizes from LOTRO's article there, do NOT make base_box_center or base_box_center_silver 1024x1024 in size. I ran out of video memory on two different PCs testing this, the engine has a very hard time cutting that thing up. I went with 32x32 I think. All the other image sizes in that article are fine.
AmbushMartyr, the best way to learn is to download a simple interface and look at the file structure and stuff. It's just images and an xml file specifying which image belongs to which part of the UI.
Note that right now fancy things like resizing elements are not possible, it is strictly just repalcing the images in the game with different images of the same size.