I haven't had the time to keep looking any further into the <picture>solution i've explained.
I hope i can found some time next week to resume this research.
Ok, here's what i've found so far and the main roadblock i've encounter also.
This "solution" is tested on iOS and android.
The only way to refresh an image inside the email i've found is using the <picture> tag.
But, here's the roadblock: In order to refresh the image you've to change the viewport, that means going to landscape or back to portrait. This way if the image is replaced on the server side while you're viewing the email and you turn your phone to landscape you'll see the new image and the same if you change again the image and then go back to portrait.
Basically each time you change orientation of the device the image is requested again each time.
But, how to force this without the orientation change action? That's the main issue i've have at this point.
I'am testing this still, to se if there's any other way to force this other than the orientation change.
Thank you all!
Ok, thank both of you for your answers.
I've actually find something that could be a starting point for a workaround.
Something that really refresh an image within the email without leaving it.
If everything goes fine in my test i'll share an example soon.