You can add a background or a specific frame to your texts or images, by adding a Widget/Shapes to your website. To do so, add a Widget block, then select Shapes. You will then be able to choose the shape you want (round, rectangle, 5 pointed star, 8 pointed star, tooltip) and customize it depending on your taste: color, gradient (you can choose its colors and orientation), shadow (this option is only available for the round and rectangle shapes).
Then you can add an image block or a Text block over this shape. The text style None (without any title nor main or side decoration) is generally the most adapted. Note that you can move your blocks to the foreground or to the background (see Move a block to the foreground)