Hello, sorry for a long reply.
It's possible to add such field, of course. But I think it's not the perfect solution because users will be able to use links to external images from other websites.
So let's try one more thing first. As I understand, you have a folder within your website where uploads are allowed, right? So you just need to set this folder as a destination folder for JBImage uploads. And disable "Upload by users" option which creates subfolders for each user. In the result all images that are located there will be listed in the select field during item submission.