Where are your SmartObject images stored?
Make sure that you are using the runtime versions of your SmartForms Urls for when you are testing and using the PDF Generation stuff for the Url string property.
I did a quick test by creating a list view on the SystemControlsFormsImage SmO then putting that on a form with a button to generate the PDF. Document attached as to how I did it. Might be worth doing the same and see what you get.
You can also include the URL Base property if you need to but I've found that is generally only required if you are making PDFs of sites that are not the Forms server.
Cheers
I forgot you are in the cloud world - you can still perform the same tests just you have to use another file control to load the PDF from using the ID.
Cheers