|Well, that's probably because the security of the folder is restricted so that other users cannot access. Home folders in the "documents and settings" directory are normally restricted for the access of Administrators, the owner and SYSTEM.|
However, why do you need to do this copy? Do you need for each user to have that file into their startup directory? If that's the case, it would be a better approach to copy the file under the "All users" folder, that way you don't even need to ask the user to run the instruction.
If you however persist in your idea, may be you could use the environment variable USERNAME and run something like
copy "..\\file\Image.jpg" "C:\Documents and Settings\%USERNAME%\Start Menu\Programs\Startup" instead, which would copy the file to the startup folder of the logged user.