You can also save the image to a PC album and use it from there like I did with my flair.
Here's what it looks like in the flair screen for me if you want to copy-paste it to experiment:
background: url('
https://www.pokecommunity.com/media/(your unique image id here)/full');
background-size: 1458px 155px;
background-color: #e8f4fe;
color: #4b196d;
text-shadow: 0px 1px #a5a5a5;
font-family: "Finger Paint";
font-size: 18px;
You can mess around with the numbers and such to get it to your liking, I just thought a 'base' might be helpful.