The header image or banner slider on your site may work differently depending on the theme (design) that you have chosen.
Adding or replacing a banner is usually just a matter of uploading the image and adding a new slide to a slider or replacing an existing image in the banner slide’s settings. It’s also usually a good idea to make sure your banner image is the right size before you upload it. For full width website designs, you’ll need a banner image at least 2000px wide. For ‘boxed’ designs (where the site doesn’t extend all the way to the edges of the browser) you can usually size your images at around 1200px. It’s best to save your image as a JPG and adjust the quality setting when saving to around 60%. This will retain image quality but reduce file size. The smaller your images are, the quicker they will load which affects how long people will stay on your site among other things.
If you require assistance getting your banner slide to appear correctly, please contact support.