How to create an online test of bias using preference judgements on pairs of pictures? I am looking to input pictures into a system to successful measure potential biases.
For example let's say I have a set of pictures and I would like the user to press either left or right once they are in the system to signify picture preference.

Is there a system available that already does this?
To answer your general question, many psychology experiment delivery systems, and even general programming languages could be used to present a set of trials where participants provide responses regarding pairs of visual stimuli. 
In particular, I've found Inquisit to be a good option if you want to deliver the experiment online. 

If you examine the task library you can find examples of IAT and GNAT studies which involve pairs of stimuli in order to estimate implicit bias. If you look around you should be able to find some examples involving visual stimuli.