Indeed, that's what I did. Didn't have to mess with the whole 4 different pictures mess.I'll give it to you right here:
Step 1. Import the loading screen model
Step 2. Make your 512x512 image, convert it to a .blp, and import it
Step 3. Change the path to FullScreen.blp
Step 4. Under the Scenerio menu, click map loading screen, and click the Use Imported File button, then select the imported .mdx for the loading screen.
IM SO CONFUSED! A) HOW TO I EDIT ITS SIZE to 512x512! How to i convert to BLP!
Check the Tools section for War 3 Model Editor. It can create a loading screen automatically. It does all the dirty work itself (resizing image etc...).