there was exactly the same suggestion some days ago, cannot find the link. the problem is: ppl already have problems understanding the current rules. there are so many signature/avatar kb size violations, i dont think it would work.
i would vote "yes" if the current system (50/70 kb) would work. 120 kb signatures (or even 90 kb sig + 30 kb avatar) would enable much better image quality - but too many users would use 120 kb signature and 120 kb avatar then.
nice intention, but i dont think it would work.