This is an example of a ready to go clickable url:

To make this clickable thumbail to put in your articles, first upload your file, then view the file and grab the url next to:

File Prview URL:

Now click the EDIT button for this article to see and grab the code below, which will appear as a box with a red x to you until you click edit:

Replace MOO-ONE and MOO-TWO with the URL you grabbed from the file you just uploaded, make sure you keep the ” marks in the code. When you replace MOO-TWO with your image URL, change the /im/ in the middle of that URL into /tn/, that will turn the url into a thumnail url for the same image.