I discovered this by accident recently and have found it very useful for checking out two images side by side which are in the same thread.
First of all make a mental note of the scroll bar position for the two images you wish to view side by side.
Place one image where you want it to be in the browswer, then remove pen (or unlick mouse)
Now place cursor at middle of scroll bar and drag to the left while keeping pen down (or mouse pressed). When cursor is far enough away so that it doesn’t affect the scroll bar any longer, move the cursor to the approximate position of the scroll bar for the other image, (do not release pen or mouse)
Now if you drag to the left when the cursor is out of range the other image will snap in to view. By moving the cursor towards or away from the scroll bar the two images will toggle in place of each other.
I hoven’t seen this documented anywhere so appologies if it is a well known fact.