User posted images creating a small problem...

[AmatsuMikaboshi]

I'm currently trying to make my own skins for my forum. I've done this many times before but these skins are something I'm trying to work on hard to release to other users, however, I've found a small problem. I like to have 'fixed' styles around 800px or so wide, which is alright until you get into signatures (while viewing PM's) and user posted images. My predicament is that I would like the whole forum to stretch along with an image that is posted past my forum's specified width. I'm not sure how to exactly do this...the most I know is that I'll have to split my banner to make a 'stretchable' part, my nav bar images will have to stretch aswell (save the buttons). If anyone would know how to do this, please let me know. I'm not real good with HTML, and I'm sure there's a relatively easy way to fix this problem.

My navbar is in the header template, if that is relevant to anything.

I've also attatched an image with an example of what I mean. The afflicting problem(s) circled in red.