Lame_Wolf - 9:20 pm on Nov 18, 2012 (gmt 0)
I guess they removed it because it is so new, and people have not had enough time to create image adverts in that size. Given time, you will be able to choose image only right from the start.
There is a lot of cost/testing involved when making a small change like that. Again guessing, but no doubt it was talked about in the meeting and was decided to just remove it from when making a new 300x600 advert. It's far quicker and cheaper to do it that way, than to also remove it from the edit advert option.