This change would be cool to apply to future versions of PP. When creating categories, you may create photo categories that you have no photos for (call it future planning). Currently, PP shows ALL categories in the drop down box when browsing, regardless if you have no photos associated with each category. Make the following change to the "functions_browse.php" page and it will correct this issue.
On line 109 where it reads this:
$query = mysql_query("SELECT * FROM ".$pixelpost_db_prefix."categories ORDER BY name");
Change to this:
$query = mysql_query("SELECT DISTINCT C1.id, C1.name, C1.alt_name FROM " .$pixelpost_db_prefix. "categories C1 INNER JOIN " .$pixelpost_db_prefix. "catassoc C2 ON C2.cat_id = C1.id ORDER BY C1.name");
From here on out, you can create all the categories you like, but they will only show up, once you've assigned a photo to a category.