you have to replace this update.img file with the one on your ps3
dont run this on your computer, your computer tries to mount this file, because IMG files are equivalent to ISO files, but this one is in a special gta game format, so its differently from standard img files, you can also open this with OpenIV if you like to add more mods, but anyway thats another story
i dont know how you like to run this game?
from the internal ps3 hdd?? or from an external hdd? or from disc?