I'm pretty sure Joshua doesn't want to criticize you, it's just a consideration: a gallery is a quite demanding project, almost comparable to ElkArte itself (depending how you plan on approaching it).
Considering you are learning php, maybe smaller projects could be more effective and help you improve your php and mysql skills in preparation for the gallery, but of course you are free to approach any project you want.
Oh, last note: a gallery requires also some knowledge of security-related concepts, not many, but doing it "wrong" will open the door to nasty people, so pay attention how you handle things in your code.
Anyway, rest assured that when the time comes, I'll be watching the code carefully.
emanuele is quite picky on this aspect.