PLZ HELP Custom Background Screen on Campaigns

Ok, I know how to make Custom Loading Screens. Export the Loadingscreen.mdx from Monolith, make the 4 .tga files in 512*512 and 512*256 Quadrants, yadayadayada.
So, I am making this Campaign and I thought, "Well, it says you can make a Custom Background Image, so I will try it out." Did it, worked and looked fine. But, when I looked at how big my 2-Map Campaign was (it will be more then 2-Maps) it was 3.7 MBS! The max size for a Campaign on www.warcraftiii.net is 4 MB. So, I loaded up another Campaign I had downloaded (Tale of the Warsong) to see how they had done it, but still had a small sized Campaign. Well, the maker had used .BLP files instead of .TGA and .BLP is like 10 * smaller. So, I surfed the internet and found the Warcraft III Image Extractor, converted my .TGA files to .BLP, and was all ready to see my Custom Screen when: It was just a PITCH BLACK IMAGE with my First Name! I was like omg... So, I come to all you great mappers now to plz, help me fix this problem and I will give you credit in my upcoming campaign. TY for your time in reading this.
://// :WD:

You need to fill the alpha channel with black. BLP supports alpha, where im not sure if its needed with TGA.