In order to have a professional looking Facebook page a banner/cover photo is a great asset. You can, of course, get away with a nice photo but something that has been custom created with the right proportions adds class.
A Facebook page banner is a very long and thin image. Many images that businesses already have may not look particularly great as a banner. I can create something like this in an hour or two that can sit on the page until the customer would like a fresh look.
The banner is an entry into the design language of the brand. The colours and blocks are features that exist on the web-page and on the stationary. This breeds familiarity with the brand, comfort. With so many competitive firms on social media everything must be done to appear consistent and clear.
The image must be designed with knowledge of mobile too. A mobile device will crop the banner/cover photo image so you must be careful to test the image to make sure no text is cut off.
Facebook pages are predominately not configurable. So, in order to really personalise a Facebook business page we need to fully use every tool at our disposal.
I started out this piece of work by asking if the customer had any photos that he particularly liked. Getting involvement from the customer is a must as he/she must be vested in the look of such an important resource as the Facebook profile page.
I created a few different versions in draft before asking for feedback. We settled on this look that melds quite well with the white background look of Facebook. The cover photo appears to very much part of the page by using white space between the elements.
For a fully professional look think about a custom Facebook banner/cover photo.