Google Ads image size 1200x628 and Sume's multiple-of-16 rule

Request 1200x640 from a GPT image model and crop 6 pixels off the top and bottom. Google's recommended Demand Gen landscape size is 1200x628, and Sume's custom pixels need both edges divisible by 16; 628 is not (640 is). The crop changes the ratio from 1.875:1 to Google's 1.91:1 (1200/628).

Google facts are from its image specification help page and the Sep 24, 2026 Demand Gen Drop post; Sume facts are from Image models, read 2026-09-30.

What does Google list for the horizontal image?

The help page's Horizontal row reads 1.91:1, "Min. size: 600x314, Recommended: 1200x628", in .jpg or .png. The Demand Gen Drop post says consumers can now visit a landing page with one tap on image ads on YouTube Shorts and Gmail, and that spec table lists 1200x628 as the recommended size.

Why is 628 a problem?

Sume's GPT custom-pixel rule is about divisibility: 628 divided by 16 is 39.25. The other limits in Image models are not the issue at this size.

Which Nano Banana setting gets close?

Nano Banana takes a native ratio instead of pixels. There is no 1.91:1 entry in the list, so pick 16:9, resize the result to 1200 wide, and crop the height to 628 afterward.

Does the crop cost anything visible?

Six pixels from a 640-pixel edge is under 1 percent, but keep text and logos off the top and bottom edges. Set output_format to png or jpeg. The same method for other platforms is in Open Graph image size.