Try setting the variable to the full path that you want (the result should look just like the samples in the FilMaker dialog (if you were going to hard code the location). Then use just the variable to tell FileMaker where to export, instead of the concatenated string.
I face a same problem, The full path doesnt work on the remote server.
Below is my set
::Xserve : Mac OSX 10.6 Server | Xserve Volumn name : ABCX
::Filemaker Server : 11 | Filemaker Server Name : ABCX5.com
I creat a click bottom and assign a script call "Export to remote server"
Go to Field [123::pic]
Insert Pricture 
Export Field Contents [123::pic; "filemac:/ABCX/Libaray/Filemaker Server/data/databases/123.jpg"]
End up the file only save in my computer (the client computer) in the Filemaker application folder.
Anyone know how to export container picture to the remote server?
I too have this exact problem! I have tried all sorts of stuff but can't get there...
Any help would be much appreciated!
I currently have a very clunky work around for uploading a container fields contents into a directory on the remote server (that hosts the fm file) using 'open URL' and some PHP scripts, but it is far from ideal, and seems it shouldn't be necessary...