Should be a field to put an image link when you edit your profile.
Discussion
but why do I have to upload it elsewhere unlike the profile pic?
No media is (typically) hosted by relays, although there's a NIP for that (95)
Clients like amethyst have several options integrated that you can pick from to upload image/media, so from a UX standpoint it works just like any other app where you upload to a central server.
tysm
