It is possible to use images hosted on Dropbox.com with JuxtaposeJS.
The most common problem is that people try to use a URL for a web page which contains an image, rather than a URL which points directly to an image. For example, this URL is for a web page: https://www.dropbox.com/s/llahmtejgebc5h5/boombox.jpg?dl=0 -- note the filename and Dropbox controls above the image.
This is the version of that URL which is appropriate for use in Juxtapose: https://dl.dropboxusercontent.com/s/llahmtejgebc5h5/boombox.jpg
The basic solution is simple: change www.dropbox.com to dl.dropboxusercontent.com in the URL. It doesn't hurt to try the changed URL in a new browser tab to make sure you got it right -- if you did, the image should show up without the controls and other content.