You can change k2 header image easily actually. Here are the tutorial.

1. First, if you never upload any images to your blog, please go to “Write”
   and scroll down until you see upload section. Please upload your images there.
2. After finish uploading your image, go to “Presentation” > “K2 Custom Header”. And the upload your own K2 header image. After that click “Update Option ” button.
3. After that scroll down until you see “Manage Picture” section. Please choose the images that you upload in previous step. After that don’t forget click “Update Option ” button.
4. Done. Now your k2 has a new header image. Need more K2 header images? My friends have it http://www.affuanramzai.com/blog/k2-header/