If you go to the gallery home page, just above the search bar there is a click clickable link that says 'upload your own Wargaming images'. Click that.
It will take you to a screen where you can select which photos to upload from your computer/phone/tablet, or as them from another site such as imgur, flickr or Facebook.
Then you have to hit 'upload' and will be taken to a screen where you can see the auto-adjusted (usually better) images or choose to use the original, enter copyright details and link to any blogs/pages that have more info on the pics.
Hope that helps, and if not, read this:
http://www.dakkadakka.com/dakkaforum/posts/list/205120.page#206792