исправлено с помощью этого кода:
public async Task<ImageSource> LoadImages()
{
var webClient = new WebClient();
var stroageImage = await new FirebaseStorage("foodymood-ba7aa.appspot.com")
.Child("Users/" + Settings.LastUserID)
.Child("ProfileImage.jpg")
.GetDownloadUrlAsync();
string imgurl = stroageImage;
byte[] imgBytes = webClient.DownloadData(imgurl);
//string img = Convert.ToBase64String(imgBytes);
var img = ImageSource.FromStream(() => new MemoryStream(imgBytes));
return img;
}