Note: Very narrow and small images can get really blurry...so it's not perfect, but it's good enough for me.

The size can be easily customized by editing the --content-width value, but you'll need a modern browser that supports CSS variables (Firefox 31 or higher, or maybe a current version of Chrome but I'm not sure).

Also, it's assumed you're using HTTPS, which is the default for Tumblr.

Update Notes:
September 7th, 2014:
- Fixed incorrect image heights