Picture size

I haven't figured out why the picture at the top of my page is small! When I created my blog, the picture was the perfect size. It filled the top part of the page (give or take) in just the right way. Ever since I changed it from that first picture the only way to get a viewable picture is to tell it to shrink-to-fit (which I think it did that first time, too, as that is the automatic setting) and now I only get those tiny pictures. It bothers me. If anyone knows how I can fix it, please feel free to pass along your advice. I would love to listen to it! :)