Image links also classify as hot linking and some sites do a variety of coding protection to prevent it. Photobucket was one such site that was used years back but they changed their structure to be more money based and you can't hotlink unless you sub up.

There's also the case of images changing after a hot link, which can be seen especially with the Gnome meme that went around ages ago, or even encoding something within the image that could potentially impact your device (example with the wallpaper that crashed Android phones a few months back)

Pretty much you just always gotta be careful with hotlinking from an outside source and when it comes to uploading from a web link, there are increased security flaws.