The problem was that an outline of the image plane would appear while using Volume Scatter to make mist. What worked for me was to turn on Premultiplied in the Images as Planes, Import Options dialog. Edit: I'm afraid this isn't the full solution as the problem still occurs, only less so. I'm thinking it may be a problem in my render settings. I'll report back here if I get it figured. [![Images as Planes dialog.][1]][1]


  [1]: https://i.sstatic.net/AQooG.jpg