The most disruptive thing about adverts is loss of the audio. The video feed shrinking is much less of an issue. Consider e.g. a Stream Display Ad that shrinks the video to 25% or even less of the screen. Advertisers would know where the stream was going to be on the screen (or even get to choose) and could incorporate that into the advert (fancy border / give the streamer a thought bubble or similar)

Additionally these could be (silent) video rather than just static pictures