Make image resizes dynamic based on page size

[bdw429s]

We resize images so they are smaller than 700 px wide and 900 px tall but that assumes a letter size output. Make this dynamic to take into account the page size in use.

The only downside of this is resolved assets are cached so once we resize them, they'd stay the same even if you change the page size. Perhaps we should store image assets with the page size in the name so changing the page size would cause the assets to be rebuilt.