Set thumbnail width & height on the loader thumbnail div css

This patch sets the height and width on the loader thumbnail div. It' a very minor tweak to stop the page from jumping up and down a bit during load. (because the image loads slower sometimes, creating the bump)

I'm working on a theme with a large ajax content div and light background, and while this is something you (should) could set in the theme's css (depending on the size of your own custom loader image if you have any), i figured why not add this to the default stylesheet for the default image.