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Yes, it is asking for source, because you need to copy and paste the URL of the image on the "Source" field.
For example, following image's URL is "http://firstname.lastname@example.org?3.3.9969". When you click to that URL, you will be redirected to that image.
Also, if the image on the web source, you can easily get the URL of the image by right clicking, then selecting the "Copy image URL" option.
After that you can paste this URL to the "Source" field in the "Insert/edit Image" on the "Autoresponder"'s template. If you still have some questions, please let us know.