This is a step-by-step tutorial on how to create a rounded corner image in PSE.

Start of by creating a new image with your preferred size. In this example I use 700x150 pixels image. Use “transparent” for background content.

Fill layer with a color of your choice. Apply effects that you like such as a gradient.

Select rectangular marquee tool.

Go to select > Modify > Smooth..

Select sample radius, click OK.

Do Select > Inverse.

Hit “Del” button once. The image will have rounded corners as below.

Close-up shot of one of the rounded corners.

Done. Edit the file further if you like and save it as PNG to use as your site’s header. You may loose the transparency of the image if saved in other formats.

Of course you can also use the same method for a footer or anything.

I’ve not tried this in PS but I guess it would be similar if not the exact same.