Go to your MTA folder. For example, my MTA folder is in my C drive;
C:\Program Files\MTA San Andreas 1.1
After getting their, double the click the MTA folder, then the CGUI folder, and then the Image folder.
C:\Program Files\MTA San Andreas 1.1\MTA\cgui\images
In it there should be a file, "radar.jpg'
That is your F11 map, replace it by simple renaming an image as 'radar' and over writing it. If you want it back as original I recommend looking through google images...