First you will need to upload the Image in Content > Photos or in Content > Uploads Misc.

After the Image is uploaded be sure to copy the URL of the Image.
Then navigate to:
  1. onMessage
  2. Website
  3. Find the Page your looking to edit the category to add the Image to (or navigate to Content > Category (Text, News etc) and find the category listed there
  4. Click the Editing Pencil
  5. Click on the HTML option
  6. Within the HTML you will enter the Following where you would like the Image to display.
    <img src="https://URL GOES HERE">
  7. Save