Vicki,
You could store these images in your Documents area within Workfront, making sure for sharing that you select make visible system-wide so any of your users will be able to view the image in the form. From there, you can use the URL of your document (clicking document details then doing right-click on the Preview button should show a dialog menu where you can choose copy URL link). This is a "live" URL so you can input in your form field. Hope this helps.