Yeah, guess what?
You can now change that boring blue cover image to a custom one right now! I don't think many people know about this....
Check out my crazy follow buttons! I used Gimp to create mine.
Here's what you can do in order to get the dimensions perfect:
Take a screenshot of your existing cover photo, then layout your art in that same file, adding or subtracting stuff as you go.
When you save your file, it will be the exact dimensions, so it will fit in perfectly for your cover image.
After you have made your art image, go to your settings page in your Steemit profile, and add your image url in the space where it says, "cover image url". I tend to use this site to upload my images: https://postimages.org/
I found out about the new cover by 
https://steemit.com/steemit/@toria/steemit-cover-photo
That's all there is to it.
Carry on,
Stellabelle