The logo is the first thing a visitor typically notices about your site, so it's important that it looks good.
The best site logos have transparent backgrounds so that the background image or color underneath them is visible. Here's a great video on how to make your logo with a white background into one with a transparent background.
Another option is a banner that stretches the maximum width of the logo element. See the Manly Ultimate Frisbee site for a good example. If you'd like to implement this option, create a banner that matches your site colors and make it 1170 x 250, which are the maximum dimensions of the logo element.