@dgroos little correction, for what ever reason it seems you need an extra option data-pat-plone-modal="image: true"
Following example works for me. You may want to add an extra style (within your css/less files) .plone-modal-content img { max-width:100%;}
to make the image fit within the overlay.
<a class="pat-plone-modal" data-pat-plone-modal="image: true"
href="http://localhost:11160/test5/ahklun_and_wood_river_mountains.jpg"
data-linktype="internal" data-val="0618ca20688f41629fe03303bedee0a9">
<img src="http://localhost:11160/test5/ahklun_and_wood_river_mountains.jpg/@@images/9cc6a06e-d492-44d0-9c89-8b963261ec93.jpeg"
data-linktype="image" title="Ahklun_and_Wood_River_Mountains.jpg"
data-val="0618ca20688f41629fe03303bedee0a9"
data-scale="mini" alt="Ahklun_and_Wood_River_Mountains.jpg" class="image-inline">
</a>