Where is the actual image file located? There's a specific folder where images must be located in order for them to be displayed via web publishing:
From the web publishing guide that comes with installation of filemaker:
"Images can either be stored inside the database itself or, for best performance, linked to a file using a relative path. If you’re hosting the database with FileMaker Pro, referenced image and movie files must be stored in the Web folder, in the FileMaker Pro folder."