Many years ago I found a really nice blog called Clover Lane and the lovely Sarah shared her tutorial on how to make a header for your blog, if you're using Blogger.
I already used Picasa to make collages so it wasn't too difficult to make one up. Fast forward 12 years and I had totally forgotten about headers until I was looking through my archives and came across her blog once more.
I immediately set too and made a header for this month. If you would like to make one of your own you can do so by following Sarah's easy instructions here. There are so many designs to choose from. Have fun.
17.2.21 - Edited to add. Headers can be made in other photo editors but the process of uploading it to blogger are the same.