Two things I do to my styles:
1) Style Manager/Main CSS: change the url paths to background images to full paths like
url(/forum/images/image.gif
2) Style Manager/StyleVars: change image paths at bottom to full paths like
/forum/images/image.gif
While PhotoPost tries to rewrite standard
vB styles, it doesnt know about all of them; so changes these values makes your style work just right without any further modification.
If you have custom items in your navbar, you would want to modify your navbar template and use paths like
/forum/script.php so the navbar can find those as well. Again, PhotoPost will rewrite the standard stuff, but if you have custom stuff you might need to edit it.