specify event logo size and dimensions

[mattstratton]

I don't know that we want to be explicit about them, but we should set a max width and then wrap the img resource with it.

This all gets better once we move to responsive/bootstrap.

[bridgetkromhout]

Okay. I think if we're recommending

 <div style="text-align:center;">
  {{< event_logo >}}
</div>

and giving no further guidance than that, then probably the default behavior needs to be non-giant size.

Example of badness:

How's this band-aid?

[mattstratton]

resolved in #220