Header images must be the desired size BEFORE adding them to your site. Resize your image with your image editing software. If you don't have image editing software, you can download a free image editor here http://www.gimp.org/ .
To add a header image:
1) Start from the Manage Pages screen. It should say "manages pages" near the top of the screen below the buttons. If it doesn't say that, click the Manage Pages button at the top of the screen and it'll bring you there.
2) Find the page you want to add an image to and click Edit for that page. This will bring you to a new page that says Edit 'such and such'.
3) Click Browse, and upload the desired document. Read about uploading a file, if you need assistance
Note:It's best to only use .jpg or .gif image documents. If you don't know what these are, don't worry about it too much. Just make sure you're uploading images rather than Word files or something like that.
4)Click the drop down list labeled DISPLAY, and select the desired alignment: ALIGN RIGHT or ALIGN LEFT.
5) Click Update Page at the bottom of this page. You will be returned to the Manage Pages screen.
Don't do anything until you are returned to the Manage Pages screen. If you're uploading a larger image or if the connection is slow it can take a few minutes for your document to upload. If you're not returned to the Manage Pages screen, your image hasn't uploaded.
6) Click Build Site.
Tadaaa...you're done. Visit the Image Trouble Shooting page if you're still having some troubles.