Figures in the introduction of a paper
I would assume that there is no formal rule about figures in the introduction (at least i haven't heard of it). Still, it would be unusual because introductions tend to not get into specifics. I'd guess it depends on the context of your paper, but if you keep it very simple and brief, you should go for it.

In case of doubt: if your paper is well-structured and well-written in addition to presenting content within the scope of the conference proceedings you aim to submit in, your submission probably would not get submitted over an explanatory figure. It could be that reviewers ask for the figure to be (re)moved though.