With 4K Download Windows 10 keeps asking what program to open jpg with. In other programs this is not a problem.

I click on picture. I get prompt asking what program to open this with. I select my preferred jpg viewer and check "Always use this app." Moments later I click another photo in 4K Downloader and the prompt appears again.
Otherwise, in other programs when I open a jpg it correctly, immediately opens in my favorite jpg viewer.