Looking through the Foursquare documentation I found the Photo Add section.
In APIs for other social platforms I have been able to post an image by its source url, e.g.: https://www.google.com/images/srpr/logo3w.png (e.g. this is possible on Facebook & Tumblr)
The documentation is a little unclear about the capabilities of the post* developer preview argument.
Is it possible to "upload" an image by its source url to Foursquare?
(otherwise can I get this added to the feature request queue?!)
It is not currently possible to upload an image by url. You will need to upload the image data with your photos/add request.