Most search engines follow the tried-and-true way to start a search. The user is presented with a long open search field followed by a "magnifying glass" symbol or other symbol or the words "search". To initiate a search, the user types the search query as text in the search box and then clicks on the search box to start the search.
If one is using bangs (!), one can then embed this in the text and then click search or press the <return> key.
Why not give users the option of a separate entry way into DDG that has different search boxes. One box says or shows the magnifying glass for "search", but other icons show "image search", another is for video search, another for audio search, etc. Which ones show up could be both generic and if one logs into an ddg account, their preferences for searching would show us.
Thus, of one wanted to do an image search, one enters the search term in the dialog box and press an icon that says in effect "image search" instead of just a regular "search". If one wanted to search for just videos, one could press "video search" or an icon showing a TV or film strip or something that would graphically represent video or movies, etc.
But perhaps this is just too much? I know that bangs are supposed to do all these things, but most newbies don't know about bangs and many appear to not use them.
Of course, maybe in the future, DDG could start its own image crawler instead of relying on Bing or Google image searches.