Although the conversion still says -1.
I've been playing around with this, and the only issue I've noticed is that the image on the Playing Now page isn't respecting the Conversion setting. If Image Conversion is enabled, the Conversion value is always sent, as expected.
The request for the Playing Now page's image uses File/GetImage with Type=Full, but the supplied Conversion parameter is ignored and the original image is returned. (Looking at the service's documentation I'd think this behavior is expected, since Conversion doesn't seem to be a valid parameter.) The Image Viewer's requests use File/GetFile, and the Conversion is respected there.
I believe the images are loaded sequentially, so if the Playing Now image takes a while to load it will prevent the loading of the images in the Image Viewer.