Avatar.cs

public async Task<bool> SetContactAvatar(string source, string identifier, MediaItem mediaItem)
{
    using (XConnectClient client = SitecoreXConnectClientConfiguration.GetClient())
    {
        try
        {
            var reference = new IdentifiedContactReference(source, identifier);

            var contact = client.GetAsync(
                reference,
                new ContactExpandOptions(CollectionModel.FacetKeys.Avatar)
            );

            Contact existingContact = await contact;

            if (existingContact == null)
            {
                return false;
            }
            
            var avatar = existingContact.GetFacet<Avatar>(Avatar.DefaultFacetKey) ?? new Avatar();
            
            avatar.MimeType = mediaItem.MimeType;
            avatar.Picture = ReadBytes(mediaItem.GetMediaStream());
            
            client.SetFacet(existingContact, Avatar.DefaultFacetKey, avatar);
            await client.SubmitAsync();
            return true;
        }
        catch
        {
            return false;
        }

    }
}

public static byte[] ReadBytes(Stream input)
{
    using (var ms = new MemoryStream())
    {
        input.CopyTo(ms);
        return ms.ToArray();
    }
}