When you first create your WordPress blog it will have a header photo (at the top of your blog page) provided by WordPress. You probably want to update that so it reflects your interests and the purpose of your blog.
You can continue to use wordpress-supplied photos or change to a photo of your own, or you can have a number of photos that WordPress will choose from randomly when your blog is displayed by a visitor.
To change your header photo, go to Appearance > Header and choose the options you want. Wordpress now offers a Customise link with header, site name and other options available with instant previews as you make your changes. Try it.
About the header photo on this blog.
My header photo is from a SOTA activation at Mt Taylor in the Woden Valley, Canberra. The Black Mountain tower is off in the distance to the north. The equipment seen on the concrete pad is a Yaesu FT817 and a IC-V85 2m fm hand-held radio. The battery on the ground is a 2.1AH Sealed Lead-acid battery (SLAB) which powered the FT817 at 5watts output on 40m, for about 90 minutes before I packed up.