All images can be right clicked and opened in a new tab to view in full size (or you can just drag them up there).
You want to create a "window" into your image. That's great! First things first, though. Familiarize yourself with Pixlr E.
This image is something like what you should see when you open a new image (we're working with an image that is 960 in width and 300 in height).
We'll be using the "Layer" and "Filter" menus at the top, the "Create Shape" button on the left menu ( ) and the individual layer settings (
) on the right side of the screen.
There are a few methods to doing this, according to how your background is saved.
If your background is an image you downloaded onto your computer, go to the "Layer" menu and select "Add image as layer...". If you saved your background as a link, click "Add URL as image...".
Adjust the image to the size and place you wish it to be. The right side of the screen should look something like this:
And your workspace something like this:
Click on the three dots to the left of the label on your background's layer and click the "Duplicate" button with two squares slightly overlapping ( ).
The Layers section to the right should now look something like this:
As you are already on the duplicate's layer, all you need to click is the "Filter" menu at the top.
Roll over "Details" and select either "Gaussian blur" or "Blur".
Drag the dot along the slider to the right until you like the amount of blur and select "Apply".