Lame_Wolf - 8:54 pm on Nov 18, 2012 (gmt 0)

There is no "image only" option on the page where you can edit the ad.

There is an "image/rich media ads only"
You cannot overtype the options.
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.