Using a javascript variable as a parameter in a java method called in a template expansion within css

I'm trying to pass a variable pulled from a javascript file as a parameter for a java method in a template expansion. The variable is the file path of an image I want to display on the web page. When I try to display the plain image, I can call it using

"${ImageUrl}"

which returns the whole file. However, the file is in an external filesystem, so what I want to do is call a method that can go fetch the correct file and handle that as necessary and pass the file path as the parameter in that method. However, when I call

"${getImage(ImageUrl}"

my java file reads ImageUrl as the text "ImageUrl".

Is there a way to call the method and pass in the correct javascript variable as a parameter?

Thanks