You mean like this?! Just figured out how to put my own picture in the game as well!
-You will need the Mixfile Remixer program and photoshop CS6 (this can be done with the trial version as well)
-You will have to download the Nvidia plugin for photoshop so it can open .DDS files
-Please make sure you have made the Mixfile Remixer compatible with your operating system or when you go to repack the files you will get an Unhandled Exception error
-Once you have extracted the GFX.MIX file from motogp 13 to your unpack folder, you can open the .DDS helmet files in the GFX/RIDER/PICTURES folder (CUSTOM_HELMETS_000_000 thru CUSTOM_HELMETS_000_019)
-Then extract any of the Moto.MIX files, go to Motogp/RIDER/PICTURES and open the SUIT_000 or which ever helmet layer you would like to duplicate, with the Quick Selection Tool select the eye to the helmet but make sure you only have whats in between and none of the suit, just helmet.
-Click redefine edge and select new layer with mask, then duplicate that layer over one of the .DDS helmet files you opened first. Save that file as is. Repeat this process for whatever helmet you would like on your rider.
-Then simply repack the GFX.MIX file with Mixfile Remixer and when you play your career just selected the helmet you saved over and you should be wearing a different model than what the generic picture displays.
HOPE THIS HELPS!!!