Update get_image_size.py

So I was actually using this while I was working on a little game project - specifically one using the Ren'Py Visual Novel Engine.

To keep a long story short, while playtesting, I noticed that this Python module worked on my Mac machines, but not my Windows machines. As it turned out, Windows *needs* the b argument (and the argument doesn't hurt the UNIX side of things); it was reading the image files in text mode.

"Python on Windows makes a distinction between text and binary files"

The module definitely made my life easier - it certainly isn't something I would have wanted to do the research for.