You need .blps, that's true. But you also ned a .mdx file.
I did it in my map like in Blizzard's map where you have to fight with new animal races (Monolith).
This map is in Warcraft III/Maps/Frozen Throne/Scenario/Monolith.w3x
Extract files which are listed below and import (after editing with e.g. Photoshop) to your map.
- LoadingScreen.mdx
- LoadingScreenBL.tga (Botom Left Image)
- LoadingScreenBR.tga (Bottom Right)
- LoadingScreenTL.tga (Top Left)
- LoadingScreenTR.tga (Top RIght)
Now go to scenario and choose 'Loading Screen'.
And check 'Imported' (or something like that). Then choose 'Loading Screen.mdx'.
This will not show the players list on loading. This will also require all players to press a key before the game starts.