It would be nice to save some bandwitdh and be able to create some more compressed images instead of the current heavy pngs that ThumbnailManager creates. Using ImageResizer I got the requests down to a third of the size by using jpgs as a workaround. A few new settings for ThumbnailManager for what type of compression to use etc would be excellent and can probably speed up edit mode some as well?
It's a common request, and would be nice to have in the platform as a standard feature, as pretty much everyone benefits from it. Happy to hear you got ImageResizer working for now.