#66 Investigate more options for JPEG thumbnail compression

Now that we're going to be working with larger JPEG images (for large-image overlay), it'd be good to look a little deeper into compressing images better. [ImageMagick provides](https://stackoverflow.com/a/11920384) (ex.) `-define jpeg:extent=300kb` which ostensibly finds a quality setting around the target filesize - the resulting file may be a bit larger or smaller in size. [jpegoptim](https://stackoverflow.com/a/25360060) apparently works better for some, and may be more specifically suited to the task, so it's worth comparing. I don't think it has scaling options, so it's probably necessary to `convert` (with 100% quality) to the target resolution first - ImageMagick provides a handy control for doing that which we use now anyway, so this isn't a bother beyond what's inherent in making each thumbnail generate in two steps instead of one.