Console graphics editing not straightforward, and a full “how-to” is beyond the scope of a forum post or two. You should start by checking the graphics section of the “Tech scroll”, to familiarize yourself with how VB graphics work.
Next, you’ll need a way to find and edit the graphics in the ROM image. My favorite ROM pixel editor is YY-Chr (https://www.romhacking.net/utilities/119/) Note: although it’s not listed as such, it does support VB pixel format.
You’ll probably also want to get a standard hex editor and learn how to use it. With some skill, it can be used to locate the addresses of particular imagery (taken from screen shots, for example) inside the ROM. I like Hexplorer, but find one you like.
Just play around (be sure to work on a [i]copy[/i] of your ROM!) and if you get stuck, come back to ask questions. Looking forward to seeing what you come up with!