I was just wondering if it is possible to make a custom background for opening and main menu for the game?
yes, there are two types of BGs somewhere in the forums.
I can't find the threads from the GP Bikes Forum to link them and to give credits, but someday I downloaded them to look if they work and they're just working fine.
https://1drv.ms/f/s!AmFynRoK33UNg_NlEjIMd6KpH25hXg
Installation:
Unzip/Copy the both bg.* files from the archive into (MX Bikes Installation directory)/misc/bg/.
You can make your own background, it just has to be a .map file. Technically you can copy any tracks .map file, rename it to bg.map and put it in MX-Bikes>misc>bg>
Might take a while to load though :)