Optimize cover images

[geealbers]

As suggested by @mandrijauskas in PR #31, the size of the cover images are okay but there may be some room for improvement. Using srcset especially for mobile seems worth doing prior to launch.

From what I'm seeing in the lighthouse test it's at 54 performance which isn't too bad. The only thing that might improve it would be to use smaller image sizes with srcsets for those resolutions, as well as using more compressed formats (webp, jpeg xr, etc) but that's getting nitty gritty performance heavy.

One of the volume authors also asked that we try to show more of the portrait when in the horizontal desktop browser configuration, so looking at varying image sizes might help there as well.