Something that was introduced relatively recently is the ability to link and update image files. So, if a png (for example) image is open in lets say Photoshop, when you save it and hit update in C2's Image Editor it will also change.
Not exactly what you say in your second paragraph, but a big step towards it.
As for the ellipses, other shapes and all the things the Image Editor doesn't do, all this and more has already been discussed many times in other topics. In short, the Image Editor is a tool that load's images, arrange them in animations, set the image points and the collision polygons. As an image making tool it exists just for prototyping stuff. I find my self making pixel art in it very often though, as it is very simple to use and sometimes I am bored to go back and forward between C2 and other software. But only for making tests and examples, not something more robust.