So the trick is that you need to have a url that ends in .png, .jpeg or .gif and at the start, you need a https://
But you should always check to make sure adding an https:// works with the link you want. Sometimes it doesn't and you just have to live with it. Imgur and tumblr seem to be the best at accepting the https:// though.
Hope this helps! Check out my ACEN adventures sometime!