You get a lot of mileage out of platform-native UI elements. Also, people have been making megabyte-sized icons these days.
It's kinda unreasonable to put restrictions on the users in order to achieve that. I mean, your slack team might not even be around by the time such displays are introduced. Why force the users to jump through hoops for such a silly reason? 90% of the time they're just going to resize whatever image they have in the quickest, dirtiest way to meet the system's arbitrary requirements.
It seems like there's a pretty simple malicious-compliance solution for your problem...
Bonus points for using http://jpegify.me/