Add width and height attributes to static image in HTML template

Hey there!

I've added `width` and `height` attributes to the `over18.png` image. I've done this to stop the annoying content reflow (therefore moving the yes/no buttons) after the page has loaded.

I don't know if there's a reason this hasn't been done before? Maybe because you'd have to maintain it if the image changed?

Anywho, :heart: reddit!