Did you install your forum into its own subdirectory (/forum) rather than into the root? That would be the cleanest, but all is not lost if you installed it into the root /. Just make sure that you install the new application into its own subdirectory (e.g., /gallery) and it should stay completely separate from your existing forum.

Assuming you're on a Linux server (Apache), there are two minor complications if your forum is installed into the root. The first (and more serious) is that whatever changes your forum put into the /.htaccess file (such as for SEO) will also be applied to your gallery. You may have to give a /gallery/.htaccess file to "undo" whatever unwanted things /.htaccess did to URLs and other things. Or, you may be able to adjust /.htaccess so that it applies the forum's changes only for the forum, and doesn't do anything if the URI includes /gallery. Either way, it's manual setup of/changes to .htaccess files, which would have been avoided if you had installed the forum into its own subdirectory.

The other complication is how you will access the gallery without going through the forum (assuming you want direct access). If the forum starts with index.php, you could write an index.html splash or landing page that has links to the forum and the gallery. index.html will be run before index.php, so visitors to the domain name (only) should see the landing page, while saved bookmarks and search engine entries to your forum should still be good. It's also possible to add a button to your forum to take visitors to the gallery, if you want to keep the forum as your main site entry.

No, there is no need to put it under /phpbb3. Put it under the root: /gallery. It will be a sibling to your forum, unless you really want it to be under the forum. And then, as everything will have passed through /phpbb3/.htaccess, you'll have to fix that .htaccess so it doesn't screw up the gallery. So, I would strongly suggest /gallery for the installation. If your forum is already in its own directory /phpbb3, so much the better.

Do you have /.htaccess redirecting visitors to / over to /phpbb3?

You can add a button to the forum to go to the gallery, if you want the gallery to be conceptually under the forum. One thing to keep in mind is that if a visitor is logged on to the forum, taking them over to the gallery may very well end up logging them out (loss of session), and vice-versa (if they log into the gallery).

Hi JohninLA, I was wondering if the site is done already. Can you post the URL here if possible? I'm trying to looking for reference in creating a site.