Thank you for your post.
When a user inserts an image as a reference, the entire path is stored. If the user changes machines, then the images will not display, since FileMaker Pro is looking for those images in a specific place. Even if you host the file on another server, when the client accesses the file on the server, the path relates to the local user. If no file is present, then it will remain blank.
Yes, FileMaker Pro will recognize the image if it is JPEG or PNG. If you are on a Mac, then PDF will show as an image, and it will show as an image on Windows if inserted on a Mac. Otherwise, it will display as a file if inserted with Windows.