AI newsletter header image: generate a 4:1 banner and crop to fit

To make a newsletter header with an image API, ask for a wide banner ratio and crop to your template's exact size. On Sume, google/nano-banana-2 accepts 4:1, 1:4, 8:1 and 1:8 on top of the usual ratios, so a header strip comes back in one call (Image API). Check your newsletter tool's header dimensions first; they differ by platform and by template.

Request

Keep the subject and any text in the middle third, because a crop or a mobile layout can remove the ends. Leave quiet space where your title will sit, and typeset the title in your email tool, not in the image.

Ask for aspect_ratio: "4:1", or 8:1 for a thinner strip. The Nano Banana 2 rate list is $0.10 per image at 1K after the 1.25 factor, rounded up to a cent.

curl -X POST https://api.sume.com/v1/images \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
  "model": "google/nano-banana-2",
  "prompt": "Wide editorial banner, paper-cut hills in teal and cream, empty calm sky on the left third, no text",
  "aspect_ratio": "4:1",
  "output_format": "jpeg"
}'

Crop math

Say your template needs 3:1 and the model gave 4:1. Keep the full height and cut the width to 75 percent of the original, so remove 12.5 percent from each side. For 2:1 from 4:1, keep 50 percent. Crop after generation so you can re-centre the subject.

If your template is much taller than the strip, ask for a taller ratio instead of cropping a wide image heavily.

File size

Header images load first in an email. Ask for jpeg or webp rather than PNG for a photo-like banner, then check the file size in your own pipeline.