Change Opacity for an Image in ImageCell

Hi. Im trying to change the opacity for Images in my ListView (with ImageCell's)
The opacity value should be based on a value in the ItemSource. e.g -> Item.UserIsOnline ? 1.0 : 0.5.
So if the user is not online I want the Image in the ImageCell to have 0.5 Opacity.
Can this be achieved "out-of-the" box with Xamarin Forms, or can anyone guide me in a direction.

Best Regards
Martin