Maybe this will help you in the right direction.
Hi Mark, I did some testing a while ago and these were the results:
Outlook 2016/2019 for Mac
iOS Native Mail App
Samsung Native Mail App
Outlook App Android en iOS
Not supported in:
Outlook 2007 and higher on Windows
Gmail App Android en iOS**
Browser support in Chrome, Safari and Firefox, not IE/EDGE. source: https://caniuse.com/#feat=apng
* Supported when using outlook.com account
Another great resource on this and images in email in general: https://codepen.io/M_J_Robbins/pen/PKXaGM
Thanks Rémi! I've found this by peeking at the html of a rendered screenshot of Outlook on Android in Litmus by opening the screenshot in a new tab and change .png to .html. Doesn't work for Outlook on iOS though.