Should a dependency on an outside paid resource be added as a core feature? Some ways I can see it, but if they were going to do that, why not just have it so that the ability can be done on the server itself. I think (I'm not a graphics expert by far) that ImageMagick and mogrify can compress the images.
And do you expect a new release of XenForo to come out each/every time they change the API? Because the very thing commented on about the add-on possibly not working will apply to XenForo until a new version is released with use of the updated API.