No problem! yeah the markdown is a pain to learn, I've been learning from a friend. Use <> Sub

_{< sub > without the spaces, and then < /sub> at the end}

If you want it centered under the image you need to get a little more intense:
< center> < img src = "Your.url.here"> < /img> < /center > < center >< sub> Image Sounce Info < /sub > < /center>

I hope that makes sense!