To post any file it suffices to click on "Manage Attachments" while editing a post, or writing a new one. You should compress it, by way of JPEG, PNG or whatever You prefer - if it's a screenshot. Zip, or rar it - if it's a savegame.
To tell You the truth, a savegame would be much more helpful - so that the character's history can be traced back.
Apparently the new forum upgrade is still being smoothed out. I've heard about issues with uploading, which should be solved shortly.
Anyway, should You still have issue You can use an external uploading service ... as long as I am able to access it and fetch the files.