When adding a Tumblr site bookmark to your Home Screen (tap Mobile Safari’s plus button to find this feature), Tumblr will serve the site’s Portrait Photo for the icon. You can tweak this photo under Customize > Info.
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Little features that could make a big difference in iOS and iPadOS
I have a growing list of little features that I hope Apple will add to iOS someday. Since WWDC 2021 was not that day, I’ll share them here.