Hi Daniel,
There isn't a built-in way to only affect the image's opacity from our control API. Can you tell us more about the usage scenario?
If you simply wanted to change the opacity of the image, you could supply the control's ImageSourceLarge property with an ImageSource value that already has the target opacity applied. The control will display the image it is provided, so if that provided image already is semi-transparent, it will render that way. This won't be animated, but would have a target opacity.
Or if you want to truly animate the image opacity with smooth transitions, you'd need to probably clone our control template and update it to support opacity animations on the DynamicImage control in the control template.