Ok, I had posted before and found out where the loading screens were stored, but when I tried changing them it did not work, just made the whole loading screen green.
So perhaps someone can help me out and tell me what I'm doing wrong here, I'm:
1. Saving picture I want as whatever.blp
2. Going into MWCS, creating new mpq archive, adding the saved blp I made to the mpq
3. As the input destination I am using rootpath of original loading screen.
+Rep to whoever helps!