Well more than likely this issue has to do with your header which has many validation errors and not Photopost. I got 53 errors just looking at your main site page and there is not much there
I beleive this is being caused the the improper placement as in opening or missing closing tag.
Photopost is pretty much xhtml compliant.
Easy way to test is real simple. Well almost simple as it looks like you have modified some photopost templates as well. If you turn off that header footer and run regular Photopost templates you will most likely find it displays fine everywhere.