Is it possible for rasa to make a judgement of "whether this input is a picture or not" and "is the slot filled" and how?

Getting Started with Rasa
2

The slot filled part is handled natively by Rasa since all slot values are available on the conversation tracker.

Re. whether input is a picture or not: This should be handled by your channel connector; the judgment that it is not text should be happening in the frontend of the channel itself.