You can certainly do this. However, it does make slide editing a bit difficult, nonetheless, it can be done. Here's how to do it:
- create a new slide using the Create a new slide using the Slide Editor option
- click the HTML Source Editor (a button on the tool bar that says HTML on it) button to open the WYSIWYG editor
- paste in following source code: <div style="background-image: url(http://www.seavn.com/wp-content/uploads/2011/04/Beautiful-Waves-17.jpg); height: 480px;">here is my text</div>
- replace the image URL (http://www.seavn.com/wp-content/uploads/2011/04/Beautiful-Waves-17.jpg) with the location of your preferred image
- replace the "here is my text" with your own text
Note: any time you need to add a line break when moving the text on the screen be sure to hold down the shift key before pressing enter