Actually, since you aren't making money off of it or using it to further your business, it isn't against the law to repost anything. It also doesn't really matter whether a meme is a repost or not, since that's what meming is all about, sharing memes you find hilarious. If you know who created the original meme, yes, give credit, but if you saw it on Google images and have no idea who created the original, you have no obligation to say anything.
And I have seen this meme as well before, but you made no mention of who created it.