I think it's time we considered adding a magnifying glass icon alongside forum images that have been reduced for initial display within a post. Clicking the icon would display the image at its full native resolution, perhaps as a popup or in a new tab. Closing the popup or tab would need to return to the same place in the forum.
Most forums do something like that, but currently it's necessary to right click (or long click on a mobile) and do a couple of extra steps to get the image in a new tab. That's unnecessarily clumsy, and if you forget to show it in a new tab, your place in the thread is lost.
Of course, it would be best if the new icon appears only when the uploaded image has been scaled down for display. so we don't waste time trying to "enlarge" an image that is already being displayed at its uploaded resolution.