To upload your pictures in a gallery you have to adquire one by participating in the monthly contests that are proposed; the five best rated drawings and one more decided by the web's creator have an user's gallery as a price. Of course, you can post your images in the gallery made for every contest opened.
Meanwhile you can post your drawings using an external image manager as Flickr, Imageshack, Photobucket or so.
You should use these sections of the forum: http://www.planearium.de/sp-studio/forum/viewforum.php?f=5
And remember not to make a post for every picture. Put all the pictures in just one post with your name and if you want to add more, just put them in sucesive replys.