Canva is a great tool for creating media for your website and also for your social media platforms (Facebook, Twitter, LinkedIn etc).
You can use Canva for free by sticking to the free elements or purchase elements for a small cost.
Sign up for Canva from their website. Choose the type of design you want to create eg. Facebook Post, Website Banner. Choose a layout and change the text for your purpose or add different elements together. You can also upload your own images to use as background or your logo.