X11 uses unsigned long int for window IDs (see X11/X.h)

src/options.c

@@ -835,7 +835,7 @@ static void feh_parse_option_array(int argc, char **argv, int finalrun)
 			opt.use_conversion_cache = 0;
 			break;
 		case 251:
-			opt.x11_windowid = atoi(optarg);
+			opt.x11_windowid = atol(optarg);
 			break;
 		default:
 			break;

src/options.h

@@ -130,7 +130,7 @@ struct __fehoptions {
 	unsigned char adjust_reload;
 	int xinerama_index;
 	char *x11_class;
-	unsigned int *x11_windowid;
+	unsigned long int x11_windowid;
 
 	/* signed in case someone wants to invert scrolling real quick */
 	int scroll_step;