Where are the files located that you are importing?
If they are not located on a shared directory accessible to all database users, there's no way to make this work.
What version of FileMaker are you using?
If you are using FIleMaker 12, I strongly recommend that you import your picture files using Import Records | Folder into a container fields configured to use external storage. And do not specify the "store a reference" option. This approach will insert the files, store them in the designated external storage location and then the files will be accessible for both mac and windows systems.
If you are using an older version of FileMaker, this process becomes more difficult due to the fact that the filepath to the shared directory will be different for mac users than that used for windows. One solution to this issue is to use Import Records | Folder to import all the records into your database with the store a reference option selected for the imported images. Then, instead of placing this container field on your layouts, do one of the following:
Put a web viewer on your layout with a calculated URL that uses the filepath from the container field, but modifies it to work with the current user's platform.
Put a calculation field with a container result type on the layout. Define the calculation to extract the filepath from the container field, but modify it to work for the current user's platform.
Note that with the method I am describing for FileMaker 11, system scripts that map and mount the shared directory can be a very useful way to make sure that the file path to the image files is consistent. Such system scripts are platform specific so you have to have one for mac and one for windows.