When hoover state is triggered the image moves to the right and down a little and I can't trace what's causing it.
I also noticed some strange behavior when changing the window size. 0-600px width - is ok. The weird stuff happens past 600px and up. When the window is enlarged pixel by pixel, sometimes it works, sometimes it doesn't. It's really hard to explain, but I'm guessing it has something to do with percentage dimensions of thumbnails inside the circles.
Issue observed on Firefox: Windows 7, FF ver. - 20.0.1, Linux, FF ver. - 19.0
IE 10 works ok.
Tested with my setup (child theme) and with original demo (http://spun-demo.calobeedoodles.com/).