The pixiv entry doesn't really explain the image, it just says "Some things are better left unknown." It's probably meant for the viewer to imagine the situation.
WillieGR said: Ow, ow~! Seems she tried to force-open her third eye. (Could be mistaken though) I know smileys are hated, but I must put this one here. :(!!
Look at the lower area of the eye. It's bleeding there. Also, I agree that I was not expecting the artist of these to do something serious like this.
I forgot to mention that the title of the image is '閉ざすとき' 'time to shut/close (the third eye, presumably)'. I'd take that to mean that the image is about the event that traumatized koishi into closing her third eye.
I think the, "Some things are better left unknown" might be referring to her ability, though I'm just taking a random guess, since I really don't know much about the character. I kinda like the image, but as soon as I saw it, I couldn't help but think of that one Ctrl Alt Del comic.