Unfortunately, I think this is a short-coming of the .mktoImg element in the email editor. Because of how it's setup, clearing the content does not also remove the container for the image and the .mktoImg component is using attributes in the HTML to "inject" the different pieces (like URL) that you'd want to easily edit.
For me, I prefer not to use the .mktoImg element at all b/c of this (and other) limitations. I prefer to use the .mktoText element instead and use the Rich Text Editor > Insert/Edit image tool which does basically the same thing.
To get the entire .mktoImg element to disappear you'll actually need to create something like a toggle where you can comment out the container all together after you clear the content. This is a two-step process -- 1) clear content 2) hide the element but this approach requires some add'l setup at the template level (an additional control to "hide" the element).
Another option would be to clone the module in the template and just outright remove the image part of it so that you've got two modules total - one with and one without the image - and then just drag-n-drop the right one into your canvas. This means you've got to migrate any content from the existing modules (headlines, links, paragraphs, etc) into the new module, but at least it won't show broken images when you clear it. Another thing to consider about this is the add'l spacing around the image (top/bottom) that also needs to be addressed when you remove the image so that there's not a wide gap between the button (above) and the headline (below).
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