Click the "Edit HTML" button beneath the theme you wish to change. The theme's HTML code appears. Search for "" in the HTML code. Move up a line or two and find "." ...

Your webpage can display a dimmed picture when users open the page, or your code can cause the image to dim later after the page loads. To make the image dim initially, set its opacity to a lower ...

You need to remove the image that is currently in your background by clicking on the grey dot in the top left corner. This will remove the image and allow you to add a new one.