Use the image editor to lay text atop your image and burn it into the image itself. Here's how:
Click on an image placeholder in your mailing, and select an image.
Click the Edit this image button.
Scroll through the selections at the top to find Text, which is the last button. Click it.
A text box will appear on top of your image. Use the dot at the bottom-right of that text box to change the box's size and orientation. Move your cursor on top of the box to be able to click and drag the box around.
Click within the box, delete the default "Enter text here" and type. Change the color options and font family.
Click the Add Text button to add an additional text box. Click the x at the top-left of that text box to remove it.