Bing in Mango will have native image search. You can see the pivot in demos when they make searches.
I also don't understand why third-parties don't get access to the search button. It's totally going against their idealology of contexual search. If an app is submitted and has a search option, it should be as easy as hitting the hardware key and there should never be any software buttons. Double clicking the search button should go to Bing as it currently does in native apps when the app searches (People Hub, Email, etc.)