You upload directly from Greenfoot.
Open your Scenario.
Go to Scenario > Export (Ctrl+E)
Fill out the necessary information under the Publish tab
At the bottom, fill in your Greenfoot Gallery username and password
Click Export
Ouh yeah, thank you very much!
In the newest version of Greenfoot, you don't even need to use the menu - there's a big share button in the top right :-)