I desperately needed sub-albums but that wasn't possible. I invented an idea which works like a sub-album. Just add links in one of your album description which redirects to your other albums or use keywords instead
Here is an example http://www.vaellusnet.com/kuva/index.php?cat=10005 (Sorry Finnish language but I hope you can figure out the idea;)
not a valid feature request, just a hint for a solution that has always been there. Marking as "invalid"