After you have completed your header in Comic Life:
1. In the file menu choose "Export - Export to Image(s)
2. Save the file to your desktop
Then, sign in to Blogger
3. Under Layout (or Template), go to "Page Elements"
4. Find your header. If your template doesn't have a header, "pick a new template"
5. Click "Edit" your header
6. "Choose" a file from your computer. Find your header .gif
7. Choose placement "Instead of title and description"