MyBB How to add a banner and make it look nice?

So I just recently switched over to MyBB for my forum, and wanted to add a custom banner to the top of it. When I did such, and resized it to 100%, it goes over the buttons which are essential to it!

Here is what I have so far:enter image description here

I want the buttons to show and I want the white in the background to be black. I have no idea how to do that. Also, I'm relatively new at web-design. If anyone could help me out, I'd appreciate it.