weston-image --help works. Also help if no filename is given

clients/image.c

@@ -412,6 +412,11 @@ main(int argc, char *argv[])
 	int i;
 	int image_counter = 0;
 
+	if (argc <= 1 || argv[1][0]=='-') {
+		printf("Usage: %s image...\n", argv[0]);
+		return 1;
+	}
+
 	d = display_create(&argc, argv);
 	if (d == NULL) {
 		fprintf(stderr, "failed to create display: %m\n");