Kobo Aura One screensaver cover low-res?

Hello,

My issue: After sending a book over, it seems to generate a lower resolution screensaver cover, regardless of the image size in Calibre.

The cover image inside the book on the KA1 is high resolution when I start reading, but the screensaver image is jagged and artifact-y, clearly upscaled.

My question: what is the best way to send covers specifically to a KA1 so they appear in high resolution on the screensaver?

My setup:
High resolution cover images in calibre library
Calibre output settings on 'tablet'
Followed the 'Shelves and collections' wiki to set up Calibre for the KA1
Kobotouchextended driver (I prefer kepub format), conversion to kepub is done on send-to-device, options selected:

• Upload covers - selected
• keep cover aspect ratio - selected (also tried not selected, no quality difference other than image stretched to fill screen)
• black and white - not selected

Other things I've tried:
- unselect 'upload covers' - no difference or worse
- manually upload covers via Kobo Utilities plugin - no difference

Anyone have any suggestions, or something I'm missing? Thanks!
~M