1) Doubleclick on CUSTOMBG.reg
- adds a line to the registry wich allows files in the wc3 directory to override the files packed in the mpq.
2) Copy the folders Fonts & UI with their content into your warcraft 3 directory
- The files in these folders will override the ingame version of these files, to undo mod simply move/remove/rename these files.
3) The background picture is located under UI\Glues\MainMenu\MainMenu3d_exp\MainMenu.tga
- To make your own background picture, you can simply replace this tga file with any other tga file.
(The picture will be stretched to fit the screen so any size will do, the higher res the better quality)
(ps. The picture _must_ have an alfa channel, so if you convert from jpg just add a completely white channel called alfa)
(ps2. ..or simply open this picture and "paste" your content over it + save)

Best of luck ingame!
kindly,
windy