As suggested by Jaff at http://encode.ru/threads/2036-Leanify?p=43423&viewfull=1#post43423

Would you like to add a small hack to optimise htm/html/css etc web based files wich contains base64 encoded images?
Detect base64 image, convert to binary file, optimise it, encode it back to base64 and replace original image in document.
It work with .png, .gif, .jpeg images. Thank you in advance!

some usefull info: http://en.wikipedia.org/wiki/Data_URI_scheme