"Let Me Tell You Why That's Bullshit" is an expression that is used to preface a counter-argument in discussion forums and image boards online.
While the reaction images are rumored to have originated from the image board 4chan, the earliest documented use of the expression online can be found in the title of a photograph depicting statues of St. John Chrysostom and St. Gregory, uploaded by Flickr user Dan Price on October 26, 2010 (shown below).
On December 22nd, 2010, an image macro featuring the caption "Let me tell you / Why that's bullshit" was uploaded to the image-hosting site chan4chan, featuring a screen capture from the anime series A Certain Magical Index (shown below). On August 23rd, 2011, the same image macro was posted in a thread discussing the anime character Bikko-Tan in the /a/ (anime and manga) board on 4chan.
On January 24th, 2012, a Facebook page titled "Now Let Me Tell You Why That's Bullshit" was launched. On November 30th, Tumblr blog “Let Me Tell You Why That’s Bullshit” was launched, which focuses on rebutting and criticizing various arguments put forth by social justice blogs. On June 10th, 2013, Redditor ElPadre62 submitted a two-pane image macro of Sudden Clarity Clarence asserting that protagonists always survive to the end of a story, followed by a screen capture of Eddy from Ed, Edd n Eddy holding a copy of George R. R. Martin’s Game of Thrones with the caption “Let me tell you / Why that’s bullshit” (shown below).
Additional examples of reaction images can be found on the microblogging site Tumblr under the tag “#let me tell you why that’s bullshit.”