OTry hide all the neutral buildings and creep camp then export the minimap preview.
After that, unhiden it all.
If I failed to understand your question, then I think you can't hide the minimap preview.
Under Scenario > Map Options there is a checkbox at the top that allows you to hide the mini-map in the loading screen. Unfortunately it replaces it with a question mark; not sure if that's alright with you though.
So you want to remove the whole minimap preview but keep the rest (the thing on the right that shows the teams and the players)? That's a tough one. I know that there's a mdx file which takes care of the loading bar; maybe there's another file that produces the team/player display.
As for removing the question mark, you could try and import a 256x256 TGA image that is completely alpha'd but the map title and the border will most likely remain.