There is a demo for a "responsive image ad" but i don't see that option when creating an ad.

That feature hasn't hit general availability (GA) yet but we just switched it on for your account.

Now, on the Ad form, there is a field at the bottom of the Creative section that allows you to target the Creative to a particular device size (Mobile, Tablet, or Desktop). There is also a link below the Creative section that allows you to add additional creatives (which can then be targeted to other device sizes).

Please let us know if you need any assistance using the feature.

For anyone else reading this, please contact us to have the responsive image ad feature turned on.