Hi there,
You can remove both the bunnies and the banner graphic at the top by going to Apperance->Customize->Site Title and unchecking ‘display header text’ (which you might want to do if you have a header image with your site title already in it).
Otherwise, you can remove the bunnies with a bit of CSS:
.site-header {
background: none;
}
You can upload a header in Appearance->Customize->Header, or if you want to exactly replace the bunnies image with your own, you can do it using this CSS:
.site-header {
background: url("your image URL here") repeat-x 0 0;
}