I'm a software engineer and long-time MMF user. I've recently started using Construct 2 and like it a lot.
As I understand it, the image editor is a "work in progress" and maybe not a priority for development at the moment. I'd just like to add my 2 cents: it could really use a few more basic tools to make game prototyping (and development) even better.
Ellipse tool---I literally spent half an hour looking for this and Googling because I was so surprised it was missing. I feel like it should be a very high priority to implement.
Non-filled rectangle and ellipse
Text (nice to have)
Polygons (nice to have, but you can simulate with lines and fill tool)
I strongly suggest making an ellipse tool a priority, followed by non-filled rectangle and ellipse, even if you don't have time for other work on the image editor. It's kind of a "first impression" user experience, for anyone using the image editor for the first time. (Btw, I know about the brush tool workaround for creating a filled circle. And I realize I can use any other image editor in existence.)
If you really don't want to invest in the image editor, a more permanent solution would be to have some simple integration with an external tool---automatically open a configured external image tool from the animation editor.