Bigger image with resize option

I must admit, I'm after some of that sweet Dogecoin bounty!

I added a much bigger doge image, and enabled a "size" query parameter.

You can do "size=300" and it will make the image a 300x300 square, or you can specify it like "300x400".

These were kind of low-hanging fruit but I'd be happy to do more later if the code doesn't look totally awful to you :)

I also added a few installation steps I needed to the readme; I'm not sure if this could be simplified by doing it in a different order, as I'm not super familiar with node/npm.