Is there a free service, with an API, that performs lossless image file size reduction?
Currently we use Smush.it which isn't perfect (have to resort to command line tools), although its FAQ does suggest that an API is coming (no progress on this in over 2 years).
Turns out the Smush.it API is alive and well, just advertised as dead. Sending requests to:
http://www.smushit.com/ysmush.it/ws.php?img=VALID_IMAGE_URL (where VALID_IMAGE_URL is, you guessed it, a valid URL of an image) returns a JSON object with a Smush.it address where you can download the newly created image. The JSON object also gives you statistics on the compression available.