xnviewmp open with gimp error message

[Ofnuts]

%U is a place holder for a variable substitution that is performed before Gimp is called. It is standardized in .desktop files. So it all boils down to what argument is passed to the probably) xdg-open command by the calling app (xnviewmp). So you problem is that Gimp is passed a directory and not a file (unless the problem is a space in the file name...)

Calling gimp %U in a bash terminal in pointless, in bash the %U is a plain string and is passed as is to Gimp.