Okular custom image stamp annotation ignores image size

TLDR: A custom image stamp in Okular is stretched to fit into a square box, apparently of the system icon size. The image size and aspect are ignored.

BACKGROUND:

One of my few remaining uses for Windows is to use PDF-XChange to allow me to stamp PDF files with an image of my signature. I use a PNG file with specific DPI and geometry to size it to about 1cm x 5cm. It has transparent background, and some use of opacity in the pen lines.

I was very excited to find out that Okular can do [custom image stamps](https://superuser.com/questions/1074614/customise-okular-to-add-my-own-annotation-stamps).

I cannot find any way to make Okular respect the dimensions of my stamp image -- it insists that every image should be the size of an icon.