First, upload the image via:
  1. onMessage > Content > Uploads Misc
  2. Copy the URL 
Then navigate to:
  1. onMessage > Styles > Edit the Style or make a copy of the Style
  2. Change the Category Dropdown to Widgets
  3. Select the Search Icon
  4. Click on the search bar and the magnifying glass
  5. Click CSS: Advanced on the bottom 
  6. Input the URL of the Image in both the Search and the Submit with Label Types
  7. Click Save
Now go to the Page and Apply the New Style to see what it looks like.