Add new cover to the blog

The previous background is getting old. Has been around for 2 years now and was still the default from the uno-theme. This patch changes the background to a new image and does some minor optimizations.

Original image from Kevin Mueller on Unsplash: https://unsplash.com/@kevinmueller

The image itself was modified in minor details such as size and format in order to be nicer to deliver to a web browser. Also the color profile was adjusted to grayscale to fit the design.