Old post but this may help someone ...

I'm guessing the jpeg images have been encoded with a stenography tool (such as steghide). As you're probably aware, stegonography is the process of embedding messages or other data within an image. Often it is difficult to see the difference between the original and the altered jpgs with the naked eye.

Tools like stegdetect will identify if the the 3 jpegs are suspected of containing hidden info. Often the tool used to perform the stenography will allow the stenographer to protect the resulting file/jpeg from being decoded by applying a password. I'd guess that is what the 'secret.txt' contains? If not then you can run another tool called stegbreak, which uses a wordlist in an attempt to break the password via brute force.

Good luck :)