I don't know how it works on iOS but on android you just tap and hold the picture and then tap on safe. does it not work the same on iOS. i have never used iOS in my life. sorry if thats a dump question.
After opening the post you need to click again (you will see the 🔍 and ✖️ top right). Then you should be able to hold and save (popup). Tested it on Safari and Chrome.