I'm trying to make an overview for my custom map before release. I followed tutorials, got my overhead screenshot. Inputed the information into a text file but the overview still does not work. The file is the right size and everything. Any ideas?

This may be a stoopid answer but did you put the two files in the same folder ???

what's the error message you get in teh console when you start your map?

You need to make sure you're using an 8-bit bmp file, which usually involves switching to indexed colour mode (256, with full-green in the last entry of the colour table if you want transparency)