With the pre-made template, you just need to select the design and elements you want. You can also add your desired image as the header picture. You can add text, shapes and images just like you are using Photoshop (but even easier for this task). It will certainly suit your need for better-designed blog / website header.
After you have designed your header, you can save it into various quality of Jpeg image. Select your desired one, then upload it for your blog or website. You can also watch the tutorial video at the website.