Sizing Header Image to fit Organic Non Profit Theme

I’m using Organic Non Profit theme for my ngo’s website. One of the theme options is to add my own header image. I can upload an image, but when it comes to the cropping screen, it only crops a limited portion of the image. Meaning the image is the wrong size. When it shows up as a header on my homepage it is too big pixelated, regardless of the dimensions of original image. How can I customize or re-size the heading so it fits properly and clearly as my header?

Also how do I make sure photos and images fit properly into theme options such as slide shows and stuff?