Hi
Whats is the file extension?? .rar??
If it is this, the map is a very big image compressed so the file is smaller and easier to download.
To open a .rar file and extract the image compressed inside, you can use the program Winrar. You can download it from here
http://www.win-rar.com/download.html
regards!
EDIT: ups! Lafrite was faster.
Any of the programs will do the work