There's a program called on xbins stop-wasting-your-time-on-meaningless-pursuits.7zip. Firstly, nobody except worthless elitists is going to judge you depending on your gamerscore, and even if they did you haven't earned a hacked gamer score anyway.
My advice to you, spend less time fucking around with savegame hacking and more time actually playing the games. If you honestly care this much about a number, you need to get your priorities straight.
As far as digging through your files goes, it isn't going to be as simple as popping a jpeg into a directory and pointing it there. Those icons are likely embedded in the file at a specific address that likely won't have any pointers or references to it. I'll admit I haven't done any hacking around with these files or savegames in general, so what I've said is pure speculation. If you haven't seen a tool specifically for that, I can't honestly say that there is likely someone on this forum who has the knowledge you're looking for. Try xboxhacker.