I've also noticed that you can drop the _image_transport:= entirely,
and just enter something like:

%rosrun image_view image_view image:=/camera/image_raw compressed

Though I'm not sure if this is recommended/supported.

image_view explicitly supports that syntax as a shorthand for _image_transport:=foo as mentioned in the docs. For other nodes you'll normally need to use the _image_transport syntax.

Patrick