Basically, the image that is going to be displayed must be available anonymously. The end user viewing it may not have the same access permissions as the user who displays the card.
For image URL, you would need to add below JSON in your card content:
{
"type": "Image",
"url": "https://developer.webex.com/images/webex-teams-logo.png",
"size": "Medium",
"height": "50px"
}
As an alternative, you can try, by first converting images to data uri, but remember the payload size should not exceed the card limit you'll be sending.
You may test the functionality here: https://developer.webex.com/buttons-and-cards-designer
Best regards,
Rudrakshi Srivastava