The Gemini guys actually think about this a lot so they're working to make the protocol the least extensible as possible or not extensible at all. They plan to freeze the spec forever after 1.0
That doesn't stop anything. There is nothing stopping me from writing up a spec that says "Put an annotation like this in your gemini file and with tools supporting this spec addition, you will see an image embedded". For non compliant browsers you will just see plain text and for new extended browsers you will see an inline image.