Allow changing image export resolution

Right now, when one downloads a graph image, the resolution (image size) is 980x400 or 980x300, so I often do a screen capture instead.

It would be nice to be able to specify a resolution (both horizontal & vertical) as part of the download selection.

I won’t add another step to the download, but if I get around to an actual settings page at some point then I could toss it in there.