To erase unwanted content from a background image, make sure that no items are currently selected. This can be done by clicking outside the canvas or pressing the esc button on your keyboard.

In the bottom right corner, the clone tool will appear--a dotted partial circle with a plus sign. Click that icon.

Click completely around the part of the image that you want to clone, making sure that your last point ends at the point you started at, to close the loop. 

Now that you have a clone of that area, you can move it or resize it to cover up any unwanted portions of your background image. The clone can be moved by clicking on and dragging it to where you want it. 

To edit the points of the clone, select the clone. Along the bottom of the canvas, three icons will appear in the center. By clicking the first icon, a trapezoid with circles on the corners, you can edit or add points to your clone. When you're finished, click the same button again, though at this point it is an x.

The second icon at the bottom center of the canvas, two lines with circles, will allow you to make edits to your clone, including adjusting brightness, blur, contrast, opacity, saturation, and hue.

The third icon at the bottom center of the canvas, two pages with a plus sign on top, is the duplicate icon. Clicking this button allows you to duplicate your clone as many times as you need in order to cover up unwanted content.

If you need to delete a clone that you've made, select the clone and click on the red trash can icon in the bottom left corner. Alternatively, select the clone and press the backspace button on your keyboard.