Help With Logo or Banner Header

When I upload a logo into the theme, for some reason the logo appears too big and when I change sizing I lose quality without affecting the sizing so much. So now I’ve decided I would like to display an image as a banner header, with my logo on the image instead so I can control how it appears. However, when I try to upload an image as a logo it doesn’t let me, when I try to change the layout of the header to allow for an image, it doesn’t let me do that either.. please help. In fact it doesn’t even let me upload a logo at all either “it just says there was a problem with your action please try and reload the page again”
Thanks