When you reply, there should be a tool bar on top of the composing window. On the second bar, smack in the middle, there's an icon that looks like a painting of a tree (INSERT IMAGE).
Welcome to Android Central! A lot of times, it's because the person asking the question is asking it about someone else's phone. This is often because the person wants to know details about another person's device without letting them know about it, which is a violation of their privacy. You should always be able to determine what's giving you a notification by swiping down the notification panel and looking at the app name associated with that notification icon (if you have the phone in hand). Otherwise, the best we can suggest is to ask the phone's owner what the icon is.