$ curl -I https://vitorpamplona.com/images/me_300.jpg
...
content-length: 29297

ignore images that are too large based on http headers content-length

ignore gifs because gif is stale file format

when adding profile pic, make client side compression to eg 500x500 webp

further optimization:
there could be two profile pic files. one small, and one larger when you open profile or click to view full size image

//

you cannot expect any specific behaviour from users. anyone can set any size of profile pic. you can however make clients not to download those.