Yes you can change your cover image on steemit! Pretty exciting right?
So what exactly is your cover image? Well it is this right here:
How do you do it? - Go onto your blog and click on settings.
This tab should appear, now under cover image you will need to upload an image 2000 pixels wide by 170 - 350 pixels in height, the height variation is based on how much info you filled in under your "about" section as the more info requires a taller image.