Err... I don't think you're supposed to insult yourself
But, if you meant 'You have the biggest pimple I've ever seen!' I'd reply with,
'You obviously haven't looked in any mirrors lately then'
Otherwise I'd just say: "That's true, pimple-boy :P" tjing-tjing
Edit: And, my new insult:
"You fight like a schoolgirl on ritalin..."