Yes, it is possible:
1, find a picture (download or make one),size = 1024x768
2, cut this picture into 4 parts

use photoshop, Model Editor tool
- TopLeft.jpg 512x256
- BotLeft.jpg 512x512
- BotRight.jpg 512x512
- TopRight.jpg 512x256
3, convert all image into .blp
4, import all 4 images, and change to custom directory as below :
UI\Glues\Loading\Multiplayer\Loading-BotLeft.blp
UI\Glues\Loading\Multiplayer\Loading-TopLeft.blp
UI\Glues\Loading\Multiplayer\Loading-
Human-BotRight.blp
UI\Glues\Loading\Multiplayer\Loading-
Human-TopRight.blp
Note: if your map use difference race (you can see ace on lobby), then you need to change "human" to one of below :
Human
Orc
NightElf
Random
Undead