The Xway User Guide (accessible from the Help menu in Xway) has a section on importing Freeway documents starting at page 189.
As Xway is a sandboxed application and Freeway predated macOS sandboxing, Xway does not have permission to access the image files that are used in the Freeway document.
The summary section describes this in more detail along with instructions on how to locate your resources en masse, thereby giving Xway the permission it needs to access the images.