How and where do i get the right size image for the header?

the height an width of an image are defined by you when you crop an image. the 'frame' they will occupy in the header is defined by the image height (ATO->Header Image->Header Image: Height)
the width depends of what you defined for a layout width. But it can be fixed or fluid, so the image needs to be as wide as the max width you specify.

as an example, my site zenofmud.org I have a max width of 1440 and a height of 250, so I make my images 1440X250

Juggledad, your zenofmud.org site is a great model. You seemed to have solved the browser-fit and screen-fit issues that I am finding to be so discouraging. I'm learning the ropes, porting my website to Wordpress and I would like to set mine up to be similarly clean and elegant, with a border. Please share your settings, if possible, set-up page by set-up page.

Sincere thanks,
Donna

To get images, you have several options. You can take them yourself. You can purchase stock images at places like istockphoto.com. If you want to find free images you can use, try searching for "creative commons images". Many of those can be used for free with attribution.

To get it re-sized and cropped to the appropriate size, use your graphics program. If you don't have one, try searching for Gimp or Paint.net for free options. You need to save the header in a web-friendly format--GIF, JPG or PNG. (If the header's a photo, you'll probably go with JPG.)

If you don't know how to use your image editing software to optimize your images for fast loading on the web, there are free online tools that do that for you. Just google "web image optimization tools" to find some options.

Here are the settings
ATO->Style & configure LAYOUT->Layout WIDTH and type (FLUID or FIXED): 96%
ATO->Style & configure LAYOUT->Layout MIN width: blank
ATO->Style & configure LAYOUT->Layout MAX width: 1440

ATO->Header Image->Header Image: Height: 250
ATO->Header Image->Header Image: Alignment: top center

hope this helps

Strange, I am now using those parameters, but take a look: Your http://zenofmud.org/ fits on the screen but my http://h23965wp.setupmyblog.com/ is cutting off on the edges -- what am I missing?

Please also explain how to make the clean box around the outside.

With sincere thanks,
Donna

try changing ATO-Header Image->Header Image: Alignment - to 'top left' you probably have it at 'top c enter'. This locks the image at one position so when you make a page smaller, it defines where the image will be cut off. Since you have it at 'top center', as you make the page smaller, the two sides will be lopped off

That worked! Many thanks!