[deleted] That whole PR is fixing a critical flaw in the implementation of the toolbar overall, not just that commit or just the image button. If you look at the whole changes for that line, you'll see that it's a big change from what currently exists. So it's probably best to be patient until that entire PR gets merged into Flarum and tested to ensure it works as intended, don't try to add that code yourself.
But yes, when those changes are tested and working, we can expect them to produce a button to insert the Markdown code for an image.
