I think for a normal user it is very unclear, what's the difference between the alt-attribute and the image legend. Even for me as a web developer, I'm not really sure.
Should the alt-attribute have a different value than the legend? In my opinion, if you use figure and figcaption tags, it may make sense to allow a different value for the legend, as it may have more content than a simple one-line-value like the alt-attribute can hold. So your editor looks ok to me, but a small info-text in the image-dialog would be helpful to new users.
I think for a normal user it is very unclear, what's the difference between the alt-attribute and the image legend. Even for me as a web developer, I'm not really sure.
Should the alt-attribute have a different value than the legend? In my opinion, if you use
figureandfigcaptiontags, it may make sense to allow a different value for the legend, as it may have more content than a simple one-line-value like the alt-attribute can hold. So your editor looks ok to me, but a small info-text in the image-dialog would be helpful to new users.