"Ayy LMAO" is an expression and Tumblr hashtag which is almost always associated with pictures of aliens.
The image of the alien has circulated the web since as early as November 2012, appearing on a number of Portuguese and Spanish-language paranormal sites including Tempo Espaço, El Gurú and El Rincon Paranormal. Though it is unclear where the photo was first posted, it was referenced on Twitter on March 31st, 2013 in a conversation between two users.
Every time I see that damn alien "ayy lmao" I lose it
— Jesse (@BoneIess) March 31, 2013
By May 23rd, 2013, the image appeared as a reaction comment on the celebrity gossip blog Oh No They Didn't. On June 8th, a Facebook fan page titled "AYY LMAO" launched, posting a handful of image macros with the phrase but not the alien photo. That August, YouTuber jorin pop created a seven-second video pairing the image with the phrase "ayy lmao" in a hot pink font scrolling across the screen.
On September 22nd, Tumblr user haunted-sugoi posted the image with the caption "this is all that matters." Three days later, the single topic blog Ayy Lmao Alien was created and began reblogging other photos of aliens with the caption from across Tumblr. On October 15th, a second Facebook fan page titled Ayy Lmao was created, this time, using the familiar alien image as its cover photo. Around the same time, a Yahoo! Answers question was asked about the meaning of the phrase. Later that month, a /r/AyyLmao subreddit was also created. As of October 28th, there are 21 Tumblr URLs, both parked and active, that use a variation of Ayy Lmao.
Additional examples can be found on Tumblr with the tag #ayy lmao.