I’m on wefwef on Android and I can just hold down on the image and the context menu has both download and copy. I don’t have an iPhone to test if it’s the same though

i don’t know about ios but some client allow you to copy image url, just copy image url and paste in browser long press on image and click save/download. If client not provide image url then copy post url and open it in browser then follow same steps from long press on image