I just made a Python script to do it. 
You can set your preferred splits too: 60/20/20, 70/15/15, etc.
It works pretty well on a small test sample and on a set of just under 200 .png files.
I used this Google images download library to get pictures, so the Python script works on the resulting directory downloads.
That reminds me, I’m using Linux. Haven’t tried it on anything else, though I don’t see why it wouldn’t work elsewhere.