On our demo, we have an image inside an HTML metabox that sticks to the bottom without any gaps.
http://new.tinygrab.com/6407caa59eb22274a735228cfd8856832554cc54a3.png
Issue: Wordpress adds P tag and that has padding on it.
Solution: We've added a little but of custom code to close this gap. Add this to your Custom CSS under theme options / general / custom css (if you've imported our demo content it should work well, otherwise you may need to adjust the class selectors to suit your layout).
.page-id-2710 #themo_html_4 .col-md-12 p {margin: 0 0 0;}
Cheers!