I've been able to use any size photo file, and so far anyways, I've been able to use any type of photo file format as well when changing these icons in Sonoma for web apps, very pleasantly surprised and pleased actually. Not sure if what I am doing differes from you, but..: Share site from Safari to dock via "Add to Dock" Right click the app icon in the dock and under options go to "Show in Finder" CMD+I on the app now shown/highlighted in Finder to pop up the Info Window With the new icon copied to your clipboard, click the old Icon in the top Left of the Info Window. CMD+V to paste the new icon over the old to update and replace. You then do have to remove the App from the dock and close the app. Re-open and add to dock again and new icon is there (For all and any that I've tried since first installed Sonoma, all remain too). I've mostly used https://macosicons.com/ for all my webapp icons. Very High quality / Free - 1,024 x 1,024 in size.