This tutorial is done in the GIMP.
Step 1.
Decide on how big you want the image to be. Next add some extra width and height to give you some room to work.
Step 2. Set your colors for the foreground and background to black and white.
Step 3
Create a new image the size that you want with the extra padding. If you do not want the outside rounded make the image fit 100%. I set the background to transparent to make the image below.
Step 4
Draw a rectangle with the rectangle selection tool. make it the size of the background image that you want. Using the bucket fill tool fill that area with the black. Next draw another rectangular area that is 10 pixels in on each side. Change your foreground color from Black to White. Use the bucket fill tool to fill the new are with White. See image above.
Step 5
Change tools back to the rectangle selection tool. Click on the image. Then Right click on the image and a menu will pop up. Click on Filters > Blur > Gaussian Blur IIR. Set the Blur Radius to 20. Should look like the image above.
Step 6
Right click on the image again and select Image > Colors > Levels. In the dialog box that opens, Enter 119 1.0 135 for the Input Levels. Click on OK to finish the process of making the corners rounded. Should look like the one below.
You could use this technique to make header frames for your header image. Just select by color the white and clear. Must have an alpha channel to to this. Since this image started with a transparent background, it has an alpha channel.