I don't think you can yet. I noticed the request was made for the blur to be added recently, so I assume it has been developed and put in place without the option to turn it off. I also notice that sometimes it is blurring the user's profile picture instead of the thumbnail, and when infinite scrolling is turned on, it stops blurring after the first page all together. It seems a bit half baked at the moment, but I am sure it will be looked at.
Image previews and thumbnails seem to have scaling (image ratio) issues at the moment as well.