Login Image Plugin – Image appears resized


I have just installed the Login Image plugin, however I have created a 310×70 image as advised but the resulting image appears resized to 274×63.

I have attached a screenshot. The black area is the image and the blue area is the size of ".login h1 a" that the background is attached to. That class style appears to have background-size set to 274×63.

Any advice on how to stop this from happening? It's annoying that the image isn't central too!

Many thanks,