Improve hero image

[lydiascarf]

revise hero image to be more proportional and provide greater visual impact (make it more visually interesting as a landing page)

I'm pulling this task out of #35 since everything else is represented in #85 and that ticket can then be closed. I think this will end up being a large enough chunk of work to merit its own ticket, particularly if we want to do a slideshow like we had in CKAN