"The correct location on the server" would be the detail to focus on here.
I must assume that you are storing these images in a shared directory on another computer? (the server)
For "store a reference" files in a container field, this requires that each client computer that needs to see those files in the container field, must map/mount that shared directory in a) an identical fashion and b) in the same fashion in effect at the time the reference to that file was inserted into the container field.
If either of those conditions are not met, FileMaker cannot find the file. Your current computer user account must also have at least "read" permission to access that folder in order to see the file from that server location.
If you have both mac and windows client machines, this gets even stickier as the two systems use different file path formats to access that shared directory and thus only one platform will be able to see the file without some additional work done to handle the platform differences.
These issues are one reason why FileMaker 12 introduces external storage as you can insert a file (do not select "store a reference") into a container field with external storage specified and FileMaker will copy the file to the external storage location and then update the container field to store a reference to this externally stored copy. The resulting file will be accessible to all client computers of both OS versions without the need to map/mount a shared directory on each computer.