Currently, you cannot use extensions in an interface.
Currently, the only way to include an image in an interface is via an attachment field or a lookup of an attachment field.
If you want to show the same image in multiple records, you can put the image in a “resources” table, link all the records to the record with the image, and then add s lookup of the attachment. You may also need an automation to link new records to the resource record. It’s a lengthy workaround, and styling options are very limited. But it is what we can currently do.
