invert logic

src/freedombone-app-gnusocial

             return
         fi
 
-        if ! grep -q "'img/custom" $gnusocial_config_file; then
+        if grep -q "'img/custom" $gnusocial_config_file; then
             sed -i "s|'img/custom.*|'img/custom.${GNUSOCIAL_BACKGROUND_IMAGE_URL_EXT}';|g" $gnusocial_config_file
         fi
     fi