You'll need to figure out the exact filepath you need. You may want to right click the folder and select Properties to help decipher the file path.
Say your shared folder is mapped to:
Then your expression for your filepath can be:
Set Variable [$Path ; value: "file:/x:/PictureFolder/" & yourtable::YourFileNameField ]
Do not include
if that part of the filename is already entered into YourFileNameField