With all these competing image formats JPG, PNG, HEIC, HEIF, AVIF, WEBP & JPEG XL it’s just a matter a time before some company or Tech Guru comes up with a solution for “fall back”. 
Test your website image performance
This was a really good article
Here’s a challenge for all the expert Bric Builders to come up with a solution for Blocs.
Implement image browser falls back.
Still no possibility to use AVIF native in blocs?
That appears to be the case.
It’s not listed here but that does say “Updated on 2nd September 2021”.
https://help.blocsapp.com/knowledge-base/asset-manager/#supported-file-formats
AVIF was officially “baselined” in January 2024
https://caniuse.com/avif
So AVIF images work in all “Evergreen” browsers, though a fallback should still be supplied for any non-Evergreen browser/versions.
When using <picture><source srcset […] <img src […] </picture> browsers will just intelligently use whatever format they support and move on. 