"Submissions must be your own; do not submit art or fiction made by others without their express written consent."
This only applies to art/fiction *on your own profile* that is not actually yours. For example, going to Bioware's website, downloading one of the wallpapers (leaving aside the question of "have they bothered making one with Ash in it?") and putting it up on your DA account, that is not something that should be submitted to the group gallery.
On the other hand, if you are browsing the DeviantArts and come across pic of Ashley and Garrus playing ping-pong in the Normandy rec room over a box of donuts (or really anything else Ashley-related that is within general DA sub guidelines), it is absolutely allowed and encouraged to click "Submit to a Group", as this sends a request to the artist for the deviation to be featured in the group gallery, and no one's work is being stolen.
To remove any confusion, policy will now read: "Do not steal other people's art and submit it as your own."