What images published in arXiv papers can I include in my education material?
Contrary to the other answer, arXiv does not require that papers are posted under CC licences. The minimal (and normal) licence required is given here, and merely grants arXiv perpetual right to distribute the article. IANAL but I very much doubt this gives you any right to use individual figures. 

You can find a (small) link to the licence for a particular preprint on the right-hand side of the abstract page in arXiv. You should see something like



If there is a CC licence you can use the image but probably need to attribute. Otherwise, I would contact the authors to ask for permission.