You will need to use an FTP (file transfer protocol) tool such as
the free version of FTP Commander or Filezilla to be able to
delete the existing images and then to post online your new versions
… with identical names and sizes to those that you've just deleted.

I use FTP Commander … see my Publishing webpage for more
info about this (including how to download it and then set it up)

Below is the image from the Publishing webpage, showing on the
left-hand side what's on my computer (these were copies of the
original WP headers that I made), and on the right-hand side,
what's online at present ... these are my substitute images of the
same size and with the same names, as the originals ...

click on this image to see a larger one

To start with, I suggest you create copies of the original WP header
images, as a back-up in case something goes wrong, and so you
can replace them back onto your website, if you so wish.

So on the left hand side (your computer side), locate and open up
your yellow folder entitled “old wp headers”. This will be empty.

Now on the right hand side, the internet side, browse for the folder
containing WP's header images...

1. click on wp-content (yellow folder)

2. click on themes (yellow folder)

3. click on twentyeleven (yellow folder)

4. click on images (yellow folder)

5. click on header (yellow folder).
   
   
   

You should now see the following in FTP Commander
(on the Rt hand side) …

click on this image to see a larger one

If you click on the top image, you will be able to highlight it in blue.
Now, press down your Ctrl key while you select all the other images.
Click on the centre arrow pointing left, to download all the images
into your empty folder ... see the resulting image below ...

click on this image to see a larger one

Now you can delete all the images on the right hand side …

Highlight them all (by clicling on them, while pressing down the Ctrl key)
> click on the Delete button on the top toolbar on the far right.

Next, to upload all your new images …

... browse for your “new wp headers” folder on the left hand side,
& open it up.

Select all the images as above, click on the centre arrow that
points to the right, to upload all the new images to the internet.

You can now go online, to check out your new header images …
these will appear in random order as you go onto another webpage,
or if you refresh the one you're looking at.