In Desktop mode there is a docker which will let you see reference images. A user pointed out that a panel to handle this is missing in Sketch mode. ref: http://steamcommunity.com/app/280680/discussions/0/558752450329427737/ Reproducible: Always
It would be cool to have something like this, though it probably should just show the currently selected image in a split or so.
*** This bug has been marked as a duplicate of bug 349459 ***