I can fix the problem by using a paragraph block containing an inline image, in place of the image block. The problem appears to be related to the fact that an image block is actually a figure containing an image (I’m speculating that’s where the extra padding comes from). With an inline image, if the width to what is desired, and then apply a custom css class containing the following:
position: absolute;
bottom: 0;
it works as desired. That’s a hack though and I’d love to know how to do it without diving into custom CSS classes if there’s a way.