that's a rap

Perhaps the commenter meant 'That's a wrap', which means 'that is successfully completed', especially used of the completion of shooting a movie scene.

so what's that's a wrap in this case mean?

I don't know; I didn't see the picture.

it's some of my drawing! it's a picture of a girl

Sorry, I still have no idea what the writer meant.

I believe the original question was correct.
rap
Meaning "Rewind And Print"
wrap
meaning "Wind Reel and Print" didnt come into print until 1974.
its meaning being assigned to the misspelled wrap for rap and a word for W was needed.
Either way it is attributed to the directors instructions at the end of filming.

I've always thought that the expression was "that's a wrap", as in "wrap things up". According to some sources, the idea that "(w)rap" originated as an acronym is false, just a popular myth, and the acronym was in fact made up later to fit the word ("backronym").