Fix height of wide thumbnail

[hadley]

Currently, the max size of the wide thumbnail is 100 x 920. But the instructions say to make an image 200 x >1000. If this is supposed to be a retina image, then the advice should be 200 x >1840. But that would require changing all existing thumbnails. I think the most prudent thing is probably just to make the displayed image on 200px high.