You can add a background or a specific frame to your texts or images, by adding a Box block to your website. You will then be able to choose the shape you want (round, rectangle, 5 pointed star, 8 pointed star, tooltip, triangle) and customize it depending on your taste: color, gradient (you can choose its colors and orientation), shadow (these options are only available for the round and rectangle shapes), rounded corners and Full screen (these options are available for the rectangle shape) and Container tool (this option is available for the Rectangle and Tooltip shapes).
Then you can add an image block or a Text block over this shape. Note that you can move your blocks to the foreground or to the background (see Move a block to the foreground)