Sometimes a photo can say more than 1000 words. Please select the best cover image related to your job. Make sure that you add a good quality photo. If you are going to Google it search for something above 1000 x 750 pixels or between 1 or 2 megas. 
In https://unsplash.com/ you can find photos for commercial and noncommercial purposes. You do not need to ask permission from or provide credit to the photographer or Unsplash, although they appreciated when possible.