Aha - if I get to Skitch by hitting the “Edit” link on the photo, and then select the tool with the ellipsis, and then select Skitch, I can mark up the photo, and then, when I hit Done, it updates the saved photo. I guess that’s the way to go for me.
I’m using Skitch on my iPad Pro and trying to mark up a photo from the Photos app. From within Photos I click on the “send-to” symbol and select Skitch, and in Skitch I mark up the photo. Then I hit “Done”. And that’s it. It doesn’t save a new or modified copy in my Photos. Nothing useful has been accomplished. I can hit “Export”, and then save my modified photo in the Files app, but is that really what I’m supposed to be able to do?