Banner, header, LxW?

Where's the best place to upload a header/banner? Click me and see where it says "Toon Wolf Industries." The reason I use it is mostly that it works out the best from my etsy, and since I could snag it, everywhere else I've tried to use it doesn't work. Here's the banner code:

<style>

main:before { background-image: url('https://i.etsystatic.com/26659833/r/isbl/80c74d/79835694/isbl_3360x840.79835694_jajrv23n.jpg'); background-position: center center; background-size: cover; display: block; content: ""; width: 100%; height: 150px; border-radius: 10px; margin-bottom: 10px; }

</style>