I'll try to explain... but remember, I am an
administrator.
Therefore, I can do just about anything in the galleries, even upload pics into other members' galleries.
Therefore, if something in this post contradicts what you're experiencing in the gallery, just say so, okay?
It's best to have everything as accurate as possible, after all.

1. Go to the
forum gallery.
2. There should be a gallery named "
Member Galleries". Click on it.
3. Every member gets a gallery when they register at these forums. I don't know how that works but it works anyway.

Anyway, search for your avatar. If you don't have one, search for your username. In both cases, click on your username. If you don't have a username, then you're not a forum member, of course.

In any case, KarjamP,
your gallery is between Kanpachi and Kushagra at the moment.
4. All members have "My Gallery" as their main category. On the right side, there should be a link saying "[Edit]", and "[Delete]". You must have a category in order to upload pics to the forum gallery, however.
If you choose to edit it, you can change the title, the parent category, the description, the gallery icon, and even how images there are sorted. Below should be an "Add Category" link that allows you to add another category.
In any case, enter it.
5. Now, you should probably be at a screen similar to this:

You'll immediately notice that I've added a red block around the "Add Picture" button - that button is glitched. Kushagra should know, he mentioned it (I think).
Instead, below, you'll see "Add Sub Category", "Bulk Add Pictures", "Add a picture" and "Return to Gallery".
"Add Sub Category" adds another category to the current category, I think. I never used it.

"Bulk Add Pictures" allows you to add up to ten pictures at once. "Add a picture" lets you add only one picture at a time, however.
"Return to Gallery" should return you to your gallery, if I remember correctly.
Does that help, KarjamP?
EDIT:
Okay, I just discovered something.
If you want to rate a picture, you first need to post a comment before you can rate it.
EDIT BY THE REAL SHANE:
Um. Basically, I'm just quoting a post from the Chess Badges threat.
This is also helpful for members who have never posted a gallery picture before, right?Assuming I'm inserting it correctly, here's the Closed Grimoire Badge.

...fail. XD I'll ask how to do that in the morning.
You linked to this page; posting the URL between [img][/img] tags wouldn't work.
Instead, open the picture in a new tab (example); that URL can be placed between [img][/img] tags.
Like so: 
You have my thanks, too: you just made me realize I ought to add more information to the Forum Gallery thread that Taylor posted.