Alot of times people paste wrong urls and then they wont understand why maybe its handy when people paste a 2000+ character url that it will say an error and that it has to end with .png or .jpg
Option to require URLs to end in image file extensionSuggestion
I don't think that the URL has to end in a filename for this to work... but it is common that users will not use a direct URL, rather a webpage with an image on it.