1) 936x220 is what I'm seeing the header image as.
yourdirtylittlesecrets.com/wp-content/themes/inove/img/header_footer.jpg (I can't post live links, I just joined here)
But that includes the navigation bar and everything. If you want to keep the navigation bar the same, send your designer that image and tell him to only design on the black area of the header and leave all the other parts how they are.

2) You can put a custom header on any website as long as you have access to the HTML files, which normally you always do. If you do need any help feel free to PM me or add me on AIM: waitintoinhale[at]gmail.com or ICQ: 587241663

3) You would have to manually edit the HTML file unless in your WP dashboard under Themes there is a link that says '(Theme's Name) Options.' Like I said, you can add me or PM me and I'll do it for free.

I hope this helps!

Yes you can install a custom header to any wordpress theme.

Here is how I do it:

To find the size of the header there are 2 ways to find it.

1) Get into the back office of your wordpress blog and then click on Appearances, then Editor. In the CSS (stylesheet) scroll down until you see the following code (below), here you will find your header size. This is from one of my wordpress blogs. See red for header size.

#header {

padding-left:0px;
padding-top:0px;
clear:both;
margin:auto;
text-align:left;
background-image:url(images/);

Or the long way is to open the images file that comes with every wordpress theme, find the blank header file open that in a program like Fireworks and the header size will be displayed.

Once you have created your new header upload it to your server into the images file within your wordpress folder. You'll find that in wp-content, then in themes, then in images.

Once you have done that change the file name to reflect your new header in the code above, I put that in blue. Click the "up date file" button at the bottom of the stylesheet in wordpress and you're good to go.

Hope that helps