At what point do I get to include images in my signature?

Never. That is disabled for all ranks.

PS: I also could not figure out how to stop that "these many posts were done since you wrote that, you now gotta submit again" annoying message.

I don't think that is possible.

You can get rid of that actually. It's handy.

Profile / look and layout preferences / "Don't warn on new replies made while posting."

Thank you very much, I knew there was a way!