1.Log in to your dashboard--> Design- -> Edit HTML
2.Scroll down to where you see tag.
3.Now copy below code and paste it just after the tag.
NOTE :
Replace "URL-OF-HEADER-IMAGE-X" with your image URLs.
If above code doesn't work for your blog,then replace "#header-wrapper" with "#header".(ID or Class of your header section).
You can add different number of images than 5.But remember to change "4*Math.random()" according to the number of images you add.For example,when you want to add 8 different images for your blog header background, then code should be change as "7*Math.random()".Look at the example below: