I found it as-cropped from an imageboard awhile back, but reverse image searching with TinEye says it's from here: http://seiga.nicovideo.jp/seiga/im3223978?track=ch2576575. I can't seem to find this picture in that link though.
I also had problems setting that as the source when uploading it at first, but that doesn't seem to be the case post-upload.