How existing users can start a new chat with an image?

Hello (again), last question (I hope).

As an existing user that did chat with the bot in the past. I do re-engaged with the bot by sharing in the conversation a new image from an other chat. 

The bot supports episode where it does asks user to send an image in order to perfom different action with it. 

BUT  in this context the user starts the conversation. So i am not sure how i can manage the bot to understand it's a new message sent by the user, specificaly an image. What should i do?:

- Create a new "re-engage" Start node? But what feature should I leverage to understand it's an image as the chat is initiated by user and it shoudln't prevent the bot to re-engage user at a different time. Also if the user start the conversation how can i set this image as a variable?

- Create a if/then node? but it doesn't support image input...

- Create a  NPL node? but same it doesn't support image input...

- Add Reset/Goto Node? but then what action should I leverage to understand user sent me an image?

At the end i want the bot to understand the user start the chat. Sends me an image that should be considered as a new variable by the bot.

Thanks for your precious help.