Hi, Jonathon! That's great feedback that I'll pass along to our product team. We'd love to hear more from you in the future but should probably cut out the middle man (me!). Mind just emailing us directly at email@example.com with any future issues or ideas?
Not really, unfortunately. The HTML
width attribute it always my override for sizing since the CSS you mentioned you're already trying to use isn't working.
Do you have to use CID images? There's an old SendGrid article on embedding images that goes into why it might be a bad idea.
Outside of that, try this on your image tag:
<img alt="" src="http://placehold.it/1600x800" width="80" border="0" style="display: block; max-width: 100%; min-width: 10px; width: 100%;">
width attribute of 80 should stop it scaling beyond that, then the inline styles take care of it being responsive but forcing it not to go below 10px wide - adjust to taste.