Static background-image "pulsates" rapidly - very strange

This is the weirdest thing I've ever seen.

I have a class of div that has a background-image, defined as such:

background-image: url("circle.png")
background-size: contain

You can see the divs here: Rouvou.com/fiction. They're called .circle-blue or .circle-red and both classes behave identically.

So here's where it gets weird: on a Lenovo ThinkCentre machine with a ThinkVision monitor, using Firefox, they "pulsate" rapidly, or grow by a pixel in width and height rightwards and downwards, and then shrink again, very rapidly, like a flicker.

Since the Lenovo is a school computer with only two browsers installed, the only other browser I could test it on was IE, in which this behavior was not found. I wish I could post video here to show it, but here's a screenshot of one of the divs:

I've tested it on almost every browser on many other types of machines, and the only place I've ever found this behavior was Firefox on a Lenovo ThinkCentre, with a ThinkVision monitor.

Has anyone run into anything like this before? What could possibly be causing it? Can anyone at least reproduce the problem on their machine/browser?

I'm using Firefox 31.0, if that matters.