Try inserting a file from that location into a container field in FM GO using the "store a reference" option. Then you can check the file path entered into this container field to determine the file path to that image and to the folder where that image is located.
If you can't insert the image, then your device does not have access to that shared directory and this would explain why you can't access that image. (GetAsText ( ContainerField ) will show the file reference text--including the file path, but also other info)
but note that you can set up a container field with "external storage" specified and this should allow both Pro and Go clients to access the image without the image being physically stored in the field.