Assuming your 12 words in the picture are actually expressed in a way that resembles captchas, Severos concerns - while are plausible and valid - are not likely in this specific scenario so I wouldn't worry too much.  Software would be pretty useless because not only are captcha solvers way less than 100%, they are terrible at finding captcha riddles in the first place in non-discreet contexts.  And I doubt anyone would bring out the big-gun NSA image analyzers for 500$USD of bitcoin.   However, if your words are plain text  then yes - someone with software will be the winner and it will happen minutes after you open it.

The delta thing is simply the technique that would be used to shorten any processing time you might try to introduce by changing the image at the last minute.

Hope this helps,