To add text slides to your video, click the Add Text button, or click on an empty slide and select the "T" option, as shown below:
A window will pop open open, allowing you to add a 40-character header, and a 50-character subtitle.
Looking to add text on top of an image? While the ability to do that isn't currently available, feel free to use photo-editing software (such as Photoshop or Picfont), to edit your image, and upload that image back into your video. Just be sure to keep your image in the 3:4 ratio, so that it doesn't get cropped.