What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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While inspecting your form I noticed the url is a link to dropbox and not an actual image url:
Actual image url:
You can see the difference here as I used the image url in one product:
I'm not sure if dropbox gives you the easiest solution to get an url. You may search for free online image hosting or you can upload the images into your Jotform account. You can consult this guide on how to do it.
But if you still want to use dropbox it is necessary for you to use a code inspector like firebug to determine the actual image url:
Let us know if further support is needed.