Messages showing "Tap to Download" instead of images
On the device with the issue, go to Settings -&gt; Messages -&gt; Send &amp; Receive and then click on your Apple ID to sign out and then sign back in again. This seems to resolve the same issue for other people.

I think the reason for the issue you are experiencing is a corrupt file, by signing out and back in your device deletes a handful of files associated with your iMessage chats and regenerate them afresh from Apple's servers.