iMac Messages is sandboxed - Confirm Service Messages wants to use the restricted Service "Open"

That's normal. First of all, you still can double-click on an in-line image in Messages to Quick Look it. Just so long as it's not already selected (grayed) and you double-click quickly. Otherwise it's just a matter of selecting it i.e. clicking on it and then, when it's grayed, hit the space bar. Either way opens it up in Quick Look. The Quick Look is not sandboxed so, if you want to open it in the default app for the file format at hand e.g. Preview for JPEG, PDF et c., then you just click on the applicable button at the top/right of the Quick Look frame that says e.g. "Preview" or "Open with Preview." The inline image thumbnail/bubble is sandboxed so, if you want to open it directly into the default app, just confirm it by clicking Run Service.

I think I've found a workaround - when you find an image in messages that you want to open, or view in its own window, just use your thumb and forefinger in a motion to "enlarge" - the image will open in its own window. You can then click "Open in Preview" along the top of the viewing window.

I'm not sure how one would do this with a mouse - I'm doing it on my macbook pro with the regular trackpad.

Hope this helps.

I have exactly the same problem and with Mojave. Can anyone help us out with this please?

Thanks for the suggestion but no luck. Have spoken with Apple support but they weren't able to help either.

Thank you "Toot Uncommon". Your answer was very helpful. Particularly the "when it's greyed, hit the space bar".

Gracious! You're most welcome. 🖖🏻

Change all PDF's with attachments to something different like SAFARI, and change all. Then get info again on PDF's and change back to PREVIEW and change all. This will also fix JPEG's as well, if you do this for those attachments.