There are few things that give a Pinterest site a greater look of professionalism than custom board covers – they provide consistency, a clean and organized look that tells potential pinners that thought and work has gone into your Boards.
They are pretty simple to create, particularly with the aid of a graphic design site such as Canva (www.canva.com). Canva offers template-driven creation of graphics.
Once you’ve created an account in Canva and verified your email, choose the type of content you want. Pinterest Board covers use a square, so choose the Social Media design – you’ll be taken straight to the content creation area. If you choose Layouts, you can select from any of the templates – some of which are free, others cost (generally very cheap). Drag and drop your choice of layout into the page creation area, or simply click on it. Change text by clicking on it and choosing other text options, and drag the text to reposition it. Scroll through each of the other elements on the left sidebar, to see what else you can position into the design – from graphics, icons and logos to free photos, or you can upload your own. Once you have designed the graphic, download it from Canva, and upload it to your website/blog.
Next add the graphic to a page on your website/blog, so you can pin it – into the Board for which you want a cover. If you can’t pin it from the page, you may need to add a Pinterest extension to your browser. On Chrome, for example, go to the Extensions menu selection (on the Window menu on Mac), find the Pinterest extension, and enable it. This allows you to pin from Chrome.
Once you have pinned your graphic into the correct Board, go to that Board – www.pinterest.com, click on your profile picture at top right, and choose My Profile from the menu (log on if you aren’t seeing this). Find the Board you are interested in, and click on Edit. 
The part that is required here is the Cover section. Click on Change, and Pinterest will show you the pins into that Board – starting with the most recent first, which will normally be the cover you just pinned. Save it, in order to get your Board cover. I also make sure that the Description contains helpful text, and the Category is set correctly, to maximize searching. I also go to the pin itself and redirect the website to my front page rather than the Board cover page, and also give the pin the same Description as the Board.
Voilà!