I signed up to a picture uploading service which when I registered an email appeared which sent me to a page on the site to authenticate the registration. On that page was an image (jpeg) with a key word in it. All the page wanted to do for you to read the word and type it into a form on the page. Apparently their service had been 'spammed' whereby a bot was continually registering accounts using them to host pictures of a dubious nature. By putting the word into an image the bot can't read it.

Sorry if the first post didn't make much sense, it was written during a particularly tedious phone conference.