Reduce image size by limiting bit depth, etc.

This optimization comes from @TylerK: we should be able to reduce our image sizes (and thus bandwidth, etc.) by modifying our imagemagick params (and/or using pngcrush) to limit the images' bit depth and remove the alpha channel.

In general, there's likely a bunch of features of PNG that we're paying for but don't really need.

The issue names no file, test, or entry point; start by locating the ImageMagick parameter handling and any pngcrush invocation in this middleware. Done means the image-processing configuration limits bit depth, removes the alpha channel, and produces smaller image sizes.

Written by the indexing model from the issue text.