Reduce image size by limiting bit depth, etc.

[rylnd]

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.

[bigtiger]

nods approvingly