Dropping a image between others in the vertical acrolling column is easier as you simply hover above a spot between two images.
Dropping a image behind the last one is more difficult and often or does but is not very fast as the target area is too small. There should be a larger drop area below the last image to give the user a larger target to shoot for. With that added all one would need is a quick flick of your finger to scroll immediately to the bottom and then a quick mouse release over a much larger target area as opposed to hunting fro the drop area.
In the first example we have a series of images with a blue center and dashed line borders representing the current situation. In order to drop a image below the last image I have to hover my mouse over that small area eblow the last image.
In the second image a blank checkered bordered area has been added to represent a larger drop area which makes inserting a image much faster.