Fascinating.
Here's what I've found on my equipment:
(Windows PC & Android, both with 1920x1080 displays, both with Firefox & Chrome)
When uploading photos to the forum...
- All images initially appear as small thumbnails.
- When clicked, images that are 1024x768 or smaller will enlarge & display in the same spot. Images that are larger than 1024x768 will enlarge & open in a pop-up window.
When inserting links to an off-site photo hosting service...
- All images initially appear as full-size, if 1024x768 or smaller.
- Images that are larger than 1024x768 will initially appear scaled down to 1024x768 high. When clicked, they will enlarge to full size in the same spot (up to 1024x1024).
- Images that are wider than 1024, *and 768 or less in height* can not be clicked on to enlarge.