If you would like to add a picture to the forum gallery then click the gallery button on the menu bar, you will now see the photo catagories. Select the catagorie your picture relates to i.e. "GAMEPLAY" then the images will open up.
Above the pictures you will see a green button "UPLOAD" click that and a new page will open up with a couple of boxes. In the box titled Image you type the title of your image then a brief description in the box below that and then upload your pic from your computer and click save.
Your image should now be in the gallery.
You can also upload pictures to your own personal album. You will see this option at the bottom left after hitting the gallery menu button.
You can create a personal gallery then using the setting if you would like to make it public or for friends only.