Using background images makes it possible to add text or logos over images without having to open up Photoshop or another image editor.
STEP 1: Select the background image
STEP 2: Choose a background colour as a fallback option
STEP 3: Adjust padding above and below your text
Choosing a fallback background colour ensures that if the background image isn't displayed, the solid background colour will be used. Be sure to choose a colour that works well with your text: for example if your text is light then you should choose a dark background colour.
Many versions of Outlook do NOT support background images:
Outlook 2016 Windows (works on Mac OS)