Gives a newly inserted Image element a built-in placeholder graphic (an inline SVG, no file involved) so it is visible on the canvas straight away, and an empty alt attribute so the image is never missing one — empty alt marks it as decorative until you write the real text. Choosing an image from the media library replaces the placeholder, and the alt attribute is filled from the media library’s alt text when one is set.
Editing
Image placeholder & default alt
New Image elements get a visible placeholder and an alt attribute.