Quoted from Dawno from another source:
If you hotlink you're 'borrowing' an image from the server where it's stored - apparently that site makes people who hotlink pay for the unauthorized use of their images by swapping out what the borrower thought you were sharing for porn...other sites have little message gifs or just block the link and you end up with one of those red x thingies instead of a picture.
So basically, you're linking the picture to this site without first uploading it yourself. It essentially steals bandwidth from the other site, so sometimes sites will swap out the picture to teach you a lesson. Now, if you merely paste the link for someone to click on, you're fine, that's not bandwidth theft. But if you want the photo to
appear in the post, you have to upload it yourself rather than link the image directly from the other website.