Gallery albums can have a “Description” that, depending on the theme you’re using, shows up under the Album name at the top.
If you Edit an album, you can make some General settings for directory name, album name, summary, keywords, and Description. The Description textbox is big and says “This is the long description of the item.”
Unfortunately, the Description only applies to (and show up on) the thumbnails page. If you’re good with tweaking templates, perhaps you can get it to show up on Photo pages as well.