Transparent background or a background image? Or both?
If you want to use a background image on a chart, then that chart needs to have two axes as those axes are used to control the width and height of the image. A text table does not have any axes, so you can't do a background image. In 2018.3, however, you can leverage transparent sheets to create a background image. To do that, follow these steps:
1) Create your text table.
2) Change the background color to None.
3) Create a new dashboard, drag on an image object, then select your image.
4) Drag on the sheet with your text table (as a floating object).