I'm not seeing the problem on v1.0.269 or v271. After tapping on the image, I can see the download button at the bottom and tapping on the three dots at the bottom right brings up a context menu that also has a download button. If this isn't the case for you, what version are you on? The version can be seen at the top of the Settings page.
v271 is the latest version so I'm not sure why it's not showing up. Maybe search for Connect for Lemmy and see if that refreshes with the update option?
Sometimes bugs are intermittent so who knows. I rarely download images so I might not even notice it. So it's good to have different perspectives.