If you're talking about a picture diagram, I think the easiest thing to do is to take a screenshot, upload the image to some free image hosting site and include a link (as I did in this question).
If you're talking about a commutative diagram, I usually make the diagram in an array (as in this question). If you need to draw a more complicated commutative diagram with bending arrows and other fanciness, it's probably easiest to resort to the screenshot method.