[NTG-context] How to use a variable in a macro agrument

I have a need to use a logo graphics file several times in a document. I somethings need to update to a newer graphics file. I only want to make one change to the original variable value definition. I've tried using \define.. but it's not working for me. Here is my MWE: %%%%%%%% \define[]\logofile{an_image_file.png} % Here defining the variable with the filename to use. \definelayer[stillimage] \setlayer[stillimage]{\externalfigure[\logofile]} % Here trying to use the \logofile macro. \setupbackgrounds[page][background=stillimage] \starttext \input knuth \stoptext What's the accepted way to do this or are macro arguments not allowable? Thanks