This post is written mainly for our customer Crossroads Lewis but hopefully others will find it useful as well.
To change the header in Twenty Eleven Theme
1] In dashboard hover over Appearance in the left hand column and click on header (or else click Appearance then header)
2] Click Choose file, then Open, then Upload, highlight the section you want, then Crop and Publish and then Save Changes (blue button at the bottom).
To have a selection of headers which randomly change for each page chosen
From the header section…
1] Upload all the headers you want using the above method
2] Next to uploaded images choose Random: Show a different image on each page. Then again save changes at the bottom.
To remove a header from the list of uploaded images
1] Click on Media on the left hand column.
2] Find the file – it will usually have a name like cropped-….
3] Hover over the name and below an option will come up to Delete Permanently – click this, then click OK
To get a particular header for a page
1] Go to the relevant page (pages on left hand column then choose the page)
2] In the right hand column towards the bottom click on set Featured Image.
3] Select the file from your computer and click open
4] Click Use as Featured Image (DO NOT click insert into post)
5] Close the window with information about the image.
6] then update the page (blue button in left hand column near the top)
To remove a featured header from a page
1] Select the relevant page
2] Below Featured Image click Remove featured image. Then update the page.