I never saw any other response to this post, so I figured I would post a followup in case someone else has this problem. A friend who has successfully installed photopost just helped me figure it out.
The issue I was having was that photopost installed correctly, with a generic black text/white background, and I could tell it was accessing the database correctly since it showed the proper login name and number of members, etc. When VB3 integration was switched on in config-int.php, then photopost would not load at all. Somehow in the process of the installation script, the forum path was specified as:
...when it should have been just
Making that change (in the integration control panel) fixed the problem. Now the gallery has the same look & layout as the rest of the forums. It's not just me - another friend who was independantly installing photopost just yesterday had the exact same error.