You should run a proxy server in front of your mod_perl server so that the large processes will not be tied up serving images. Take a look at the information here for help on how to do this.
Comment on Re: 2 Questions on Images and Mod_perl mem usage
perrin++. This is huge. You might be better off with 1500 apache processes and 150 apache + perl processes, rather than 700 apache + perl (numbers depend on situation). The danger with any mass-market application is that you're going to have a bunch of your bulky processes waiting around to push those 2 MB images down a 56k line.