I am using png images and the Images as Planes add-on to create background trees for a scene. This works fine until I try to combine this with Volume Scatter for a mist effect. The alpha portion of the images lightens, showing the rectangular outline of the image against the sky (hdri). I'm using Cycles Render. I have the density set to .02. If I set it to .005 the effect is not too noticeable, but the mist effect is minimal. Edit: I may have figured out a solution. In the side box of the Add Images as Planes add image window there is an option called Premultiplied, when I chose that, the effect seems to go away.
The outline shows up against the sky with the Volume scatter set at .02. It appears that where the two alpha images overlap the effect cancels out. Maybe a work around would be to add a large alpha plane over the whole scene. The trees with no volume scatter.