Fix some image handling-related variables in backend

backend/pt/action.json

@@ -96,6 +96,6 @@
         "online_usage_museum": "Online usage (museum)",
         "stats_instance_contents": "Database contents of the instance",
         "video_conference": "Video conference",
-        "image_choose": "Select an object ..."
+        "image_choose": "Select an image ..."
     }
 }