Thank you for reaching out to Constant Contact API Developer Support, and for your patience. My team is here to assist outside software developers with questions about building into Constant Contact's API.
It looks like your image URL may be trying to include the image file in the payload. You’ll need to host the image somewhere and then link to it. For image hosting, you can use your Constant Contact Library, your website, or another hosting service that is able to provide a compatible link for the image to be linked from.
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