I see... this is getting into a case the API didn't really decide upon specifically, but my intuition would be to expect 3 methods: `get_extinction_image`, `get_100um_image`, and `get_ebv_image`.  Or, `get_image_type(type='ebv')`, but I like the former approach (3 methods) because "discovery" is easy (i.e., tab-completion in ipython will show you what types are available).  I don't think this approach is universally best, though - we don't want `get_error_image`,`get_coverage_image`, etc. methods for raw data services, so perhaps the dust service will prove unique in this way.