Thanks, Scott! I had started drafting up this edit to my reply, so I’m going to go ahead and post it then take a look at your image links. 
When creating the Media Item in Manage Resources or adding a Media Item as a background in a Theme, you should see a preview of the image displayed. If the preview doesn’t display, it’s likely not going to load as the background either and indicates some sort of issue.
Totally agree. At one point in time, browsers exposed information about the reason why an image didn’t load, but most modern browsers don’t expose this information anymore for security purposes. That being said, if you are on a laptop or PC, you may be able to open the console for your browser and get a clue as to what went wrong. (eg. F12 on Google Chrome)
The two most common reasons are either the image truly isn’t available (eg. typo in the URL, server can’t be reached, etc) or more recently that Google Chrome has begun blocking mixed content (eg. the image is HTTP instead of HTTPS).
Keep in mind that both Dropbox and Google will serve images up in a webpage rather than serving the raw image by default. For dropbox, you can change the ?dl=0 to ?raw=1 to force it to serve the raw image. The How To: Add photos to a dashboard post has some tips on images from Imgur and Google Photos.