Step 1
Let's first create background of the blog. Inside the Header group draw any color rectangle shape using the Rectangle Tool (U). In my case I drew 1200x600px size rectangle and placed it at the top of the document.
Now download Cafe photo, drag it to the Photoshop document and place it above the rectangle layer. Rename the image layer to something you can recognize later, in my case I've used IMG. After that hold down the Alt key and mouse over the photo layer until you see a little arrow pointing down, then release it. You have just created a Clipping Mask.