Facebook Cover Quality

Hy guy.
I think this is the place to ask some questions. Can you help me out?
So i just started to make facebook covers for my items, so the buyer will make the most out of his purchase.
But when I try to preview it on Facebook, the image is so blurry and distroted.
What am I doing wrong? I know it should be less than 100kb so it won’t compress, but… still
Can you help me out with some tips.
Thank you very much