It's a play on words, see. The phrase "out of my league" typically implies that someone is either too attractive to date the speaker or otherwise well overqualified for a job or other situation. In THIS instance, though, I decided to apply the term more literally than figuratively by posting a picture of someone who is in a professional sports league while I am not in the league. Thus, she is "out of my league."
I mean in the figurative sense too it still applies, however the literal interpretation was the point of the joke.