Add png images, update spec test

[hubdotcom]

Final file size check: river.jpeg 226,169 bytes

river.png 167,796 bytes

A 25% reduction in filesize and enhancement to lossless quality images.

Note: Fast compression reduces the filesize to 155,609 bytes but almost doubled the test times

[stephenyeargin]

Images look sharp, and I pulled down the branch to verify that it passes all of its specs. :+1:

[CharlieHess]

Legit, good work @hubdotcom.