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There is no "image only" option on the page where you can edit the ad.There is an "image/rich media ads only"
I've not tried it, but you may be able to use the old-style code, and overtype the image size - where needed - and have it set to image only.
I guess they removed it because it is so new
but it has done better than the 300 x 250 that used to be in those spaces has typically done