1. Drag the image Element onto the custom form canvas
2. Select the image Element under Values select Value Source as Constant and add the URL for the image to the Value field.
If the image is blank or does not load then its likely there is no connectivity to the site where image is hosted.
Optional , if network access is not available to the site where the image is hosted you can add a base64 encoded value for the image.
To convert the image to a base64 string. There are online tools to do this for you, some of which are:
https://www.base64-image.de
https://base64.guru/converter/encode/image
The Data URI should be in the following format:
data:image/png;base64,{base64 image value}