"Picture background color" set to "Auto"

It will affect the transparent background of the PNG image, which will result in the inability to view a single color PNG file.
If it set this on,
1
will show like this
2
This obviously will not be the result I want

Can't tell for sure without a copy of the PNG (the post above has a screenshot of it) but I'm guessing the PNG has only one color, black, and the top-left pixel is black but with 0 alpha.

I'm not sure what auto should do in that situation. Maybe you should still be able to choose a background color to be used when the image has transparency, and "auto" should have no effect on images with an alpha channel.

Auto show like this in this situation will be better :joy:
4

The ability to attach your own image as a background to the standalone and ordinary viewer in Opus would be a very desirable feature.

If that image were a checkerboard -à la Photoshop's transparent background indicator - It would immediately indicate visually whether the image been viewed was masked or not.

Currently the best you can do is set the background of the viewer to white, which is hardly satisfactory as, the image may simply have a white background.

I use a checkerboard background on all my image listers and can tell at a glance which of my images have transparent backgrounds.

You don't have to choose white. I use a color that's unlikely to be used as a background.

A checkerboard for transparency would be nice, and is something we might add in the future, but not easy with the current viewer code (at least to do it properly, so the checkerboard doesn't move and scale when the image on top of it does), so probably not in a point release.