For the record, there are many useful links and tuts around here (this forum & others). Will add more.
Hiding Chat Messages
About Minimap preview
Use a MPQ Browser like Magos War3 Model Editor, right click a open source map, press Open With war3modeleditor, on the mpq browser, search for the minimap file and open it. Use the texture manager to know the size of the minimap.
Minimap size is variable according to your mapsize.
After you know the minimap size, resize your picture, and import as war3mapMap.blp