Make it an option to scale images to fit the page.
Resizing hundreds of pictures manually is frustrating, whereas Anki scales them to fit the screen perfectly.
I'd pay for this feature in Mnemosyne.
I was very annoyed, when I naively used the Windows snipping tool on a 4k monitor and now can't use the cards anywhere else, really.
-New command line option "--high-dpi-scaling" to enable scaling for displays
with high DPI values. Not enabled by default because it can cause some
issues with multiple connected displays.
I'll chip in a bit of money too. This is especially important on devices with smaller screens.