As DC has feature to re-send any message (for example, to solve some multi-device issues), it would be better to similarly re-send profile images. At now some devices can miss avatar changing, and there is no option for update contact’s avatar.
To have a button in profile settings to re-send current avatar for your contacts.
New avatar is sent to contact once. So some devices may missing this update, and would show old or absent avatar.
in practise, it should no happen that often that a profile picture is missed (apart from bugs 
) - and if it is missed, for whatever reason, it is automatically resend every some days, so that over time, things should get fixed automatically
so, an explicit option for that should be weighted carefully, in general, we’re not friends of options and the like, trying to avoid all complicated questions and decisions
however, maybe we can force resending the profile picture when a message is marked to be resend - the resending as such is already a strong hint that something is missing for some recipient
I have already suggested implementing an algorithm where an empty list is created after changing the avatar and each time a message is sent, it is checked whether this address is in the list, if not, then he needs to throw a profile photo and write it to the list. After changing the profile photo, the list is erased again. So this is an ideal algorithm.
Your suggestion for implementing profile picture re-send as a part of an option to resend message is good. In spite of automatically re-sending profile picture, sometimes it is better to force this without waiting. Our stateless communication should have an option for recover coherence.
My example, I have multiple devices and setup deletion of mails in a server after 1 week. If I don’t use some device more than 1 week and profile picture had been changed, on this device I won’t have a new profile picture, and I cannot fix it now.
You solution is not reliable for multi-device usage. Some devices may miss profile update. See my example above.
When I meant the list, I meant that my address should also be there, it is sent immediately when changing the profile. On other devices, an email with a new profile picture will be processed according to the order. The algorithm works, I checked it personally, everything is perfect.
I’m sorry, I misunderstood. You’re right about that. I’ll figure something out.
And what if every time you send a letter to the interlocutor through the header, you send an md5 of his profile photo, and when reading the letter, he will compare this md5 and if they differ, send a letter about reading the profile photo. And the devices that are in sync will pick up these outgoing emails about respect and act the same way. If I made a mistake somewhere, please correct me.
And it is even better to exchange exif (date and time) rather than md5. And whoever has it later throws a profile photo. Thus, the profile photo can be updated on the second device thanks to the interlocutor who stores your profile photo.
You propose an interesting mechanism. Yes, such a solution can solve the described issue. How to generate such a fingerprint for a profile photo is a matter of discussion, however your proposal will work regardless of a generation method. I think that a simple timestamp of profile change is just enough.