Do you want to add a new image for your blogger's header, you can do it by following the next instructions.
1- Login to your Blogger account.
2- Select "Layout".
3- In "Page Elements" section you can edit "header element".
4- Pick your new banner and save it.
You can decrease the size of your current banner by creating a smaller image to be the new banner. If you want to see an example just look at my banner and observe the small size especially for the height of the banner.
You can use PhotoShop programs by Adobe to create a good banner for your blog. You can also edit the old image to be more unique. By the way, visitors like the unique blogs that seems to be different from other blogs and a unique banner will do this for your visitors. Happy Blogger