Resolution & Finding Images of a Decent Size
Now finding an appropriate image is another story. The video below describes resolution and how to use Google to find an image that will work for you. watch it and then you can go look for your own image on the web.
If you can't find images that are suitable when you do an advanced image search for large, you can drop it down to medium. Just keep in mind that if you do that, the images are going to be smaller yet!