This workaround seems to work for me: (using my Windows 11 desktop computer)
1) Create all the notes you want, ignoring the thumbnails.
2) Create a brand new note and paste the image you want to use as a thumbnail. (I wait for the thumbnail to appear in my Snippets view.)
3) Select all the notes you want to be with the thumbnail note and merge them, with the thumbnail note moved to the top note to be merged.
I still want the thumbnail selection to be native to EN.
--- A L ---
As an aside, my 300dpi jpg's seem to work OK, but my 600dpi jpg's do not show. I have not tried any values in-between.