Converting uiimage to stream?

i am using buddy sdk

client.CreateUserAsync("username","password").ContinueWith(r => { var user = r.Result;

// upload a profile photo
Stream photoStream = GetSomePhoto();
user.AddProfilePhotoAsync(photoStream);

});

this sample tells me how to add profile picture, i try with every way but when i try to use this code

        var img = loginImagePicker.Image.AsPNG().AsStream(); 
        var hu = await user.AddProfilePhotoAsync(img);

it gives me over array problem

any advice?