In using Canva (or InDesign, et al.) to create your own background, here’s a tip to get better screen resolution. In Canva, don’t download your file; in INDD (or other graphics tool), don’t export it as JPG/PNG. Instead, while maintaining its aspect ratio, display the image as large as you possibly can within the application (I use a 24″ monitor), take a screen capture and then save that as a JPG/PNG. Once that is uploaded to LinkedIn, the resulting JPG/PNG may have to be scaled down. This is a good thing, as it will actually enhance the resolution.
Whatever you decide, make sure that if you borrow images for a website background , the images are either creative commons, or that you own the right to use them. Make sure you follow the rules of any creative commons license that you take up. It is not OK to just download an image and use it without getting permission first. If you use something that you do not have the rights to then at best you could get a takedown notice, and at worst you might get taken to court. You will, either way, end up looking unprofessional to the people who find out that you ‘borrowed’ images without permission.