The correct way to define this custom label is _IMAGECustLabel.

CustLabel_IMAGE and _imageCustLabel are incorrect ways of defining the custom label.

This field is case sensitive, so the prefix “_IMAGE” must always be added in upper case.

Images will not be rendered when this prefix is not used in the custom label. The Integration App supports rendering images in the HTML Template from a custom record in NetSuite when the custom label you wish to use has “_IMAGE” as a prefix.This feature only allows you to use an HTML template for importing item information from NetSuite to eBay. Note : Celigo doesn’t provide an HTML template. This feature allows the seller to dynamically replace these placeholders or tokens with product attributes/information from NetSuite before the listing is created/updated on eBay. Usually, these HTML templates have place-holders or tokens to hold the listing specific information. The emphasis of this feature is on enabling you to use the same HTML template for multiple product listings, yet display information that is specific to a product in your eBay store. Most sellers create 1 or more HTML templates for listing their items on eBay. This feature helps sellers to enhance the way their product looks on eBay and to be consistent in terms of branding. EBay provides its sellers with an option to use HTML templates for listing items, where sellers could provide a rich HTML content template (on an item level) and fully control the appearance of the listing on the eBay front end.

Ebay item descriptions css template