I have a layout that is slimmer than the regular default layout. What is a good code to stick into my layout code so that my friend's posts (usually with pics) will be slimmed down to fit my layout?

i don't think there is one.
its a problem on most peoples ljs. the page will stretch. sorry.

Yeah, people are rude sometimes about that. If my image is bigger than 300 pixels in width I either thumb it or put it behind a cut so it doesn't stretch other people's friends pages.

Actually, there is an option now that I think of it.. Go to edit your info settings, and towards the bottom there is an option that says "Use image placeholders on your friends page". Check that box and in the dropdown menu you can choose to make all images turn to links, all over 320 in width, and all images over 640 in width. That will stop them from messing up your friends page. :)