Update Email Content Image with API

Hello,

I'm trying to use the API to update an email template's banner image, which has a type of "mktoImg." The API documentation says that “Text,” “DynamicContent,” or “Snippet”  can be edited. When I tied the type of Image, it returned an error, saying "Cannot find an image with this id." The issue is that I'm trying to include an image with an external url, not one with an ID inside Marketo. Is there a way to update an email's image field with an external URL? I thought about using a variable instead, but then it would no longer be editable in the editor.

Thanks.