The variable exportPath is what you want to change near the beginning. It should read:
var exportPath = "~/Desktop/Backdrops/png/"; exportPath = exportPath + '/layers';//Get rid of this
The line you mentioned above should be:
desc4.putPath( id25, new File( exportPath + fileNameBody + ".png") );
Again, you have to be logged on as "Tim" to acess that user's folder. If you're not, you have to run the script as an Admin and then spell out the entire path.