Girls on /r9k/, has a boy ever seen you naked when you didn't want him to? Were you embarrassed?
The joke is that the character OP posted, Akari, is often times considered invisible or transparent on the show. >>24062133 is pretending he cannot see the image, because it features Akari.
Akari is a character from the anime Yuru Yuri who is known for having little presence and being easy to not notice. The joke is that her picture is being mistaken as no image. Watch more anime you baka gaijin.