When building HubSpot themes you often want to have default images referenced from inside your theme folder. You would think adding a relative link would work but you actually need to add it inside this hubl function to get it to work get_asset_url()
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"default" : { "src" : "{{ get_asset_url('../images/your-image.jpg') }}", "alt" : "", "width" : "1440", "height" : "683" }