Add Text Layers to Your Slide

From the slides list, click the green "Edit Slide" button to the left of the image you wish to place a text layer over

 

On the next screen, click the "Add Layer" drop-down

Select "Text Layer"

 

Add your desired text in the text box to the right

Click and drag the text box on the slide preview to the left

 

You can configure the color and display of the text layer to the right under "Style"

Be sure to click the green "Save" in the upper right hand corner and flush your cache to see your work on the live site right away.