Click on Appearance / Header in the left sidebar of your admin page. Find ‘Choose an image from your computer’ and click ‘Browse’. Search your computer for the image that you want and then click ‘Upload’. You’ll then be taken to a new page which will show you your image.
On top of this image is a white box. You can ‘crop’ the image, meaning select the bit that you want to publish on your website, by pressing your left mouse button as you ‘drag’ one of the little white squares on each corner. You can also drag the whole box around and position it over your image by putting your mouse in the middle of the white box as you press the left mouse button.
If you don’t need to crop your image click ‘Skip cropping, publish as is’.
Once you’ve uploaded more than one image you also get an option to randomly display those images.