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.
The support for saving images was added yesterday. It probably will be developed further. I agree on that it would be nice to be able to save an image without going into the post.