Making a gradient for the background of your webpage. Start in Photoshop. Make the width whatever size you want, and the height the length you want the gradient to span in your page.

Go ahead and make a gradient through your Gradient editor the two colors you want, one color being your web page's background color.

Select the gradient with your Single Column Marquee Tool.

Image/Crop.

Save the file as a .jpg in your site's images folder. Insert either in your Page Properties or in your CSS. Make it your background image and set Repeat as "repeat-x", then set your background color as the bottom of your gradient.