loop

var sizes = {
	'-small' : 200,
	'-medium': 600,
	'-large' : 1040,
}

for (var suffix in sizes){
    if (sizes.hasOwnProperty(suffix)) {
        var finalName = uploadUrl+fileName+suffix+fileExtension;
        var stream = fs.createWriteStream(finalName);
        gm(tmp_path)
        	.setFormat("jpg")
        	.resize(sizes[suffix])
        	.write(stream, function (err) {
        	  if (err) throw(err);
        }); 
    }
}

var finalName = uploadUrl+fileName+fileExtension;
gm(tmp_path)
	.write(finalName, function (err) {
	  if (err) throw(err);
});