In the Template editor, just click on Media and then Pexels. Type in a specific keyword to narrow your search. For example soccer, tennis or kids. Click on a photo and it will get inserted in your template.
Pexels presents themselves as "the best free stock photos & videos shared by talented creators". You can become a creator too. I invite you to check it out. If you are a photographer, you can contribute and enable others to send beautiful emails.