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Why pictures are not showing for items that listed via APIs when the Item's pictures are set to self-hosted URLs
There can be several reasons for the item picture/image issue. One of the reasons for the issue can be, the self-hosted picture URLs is with 'HTTP' protocol.
Self-hosted pictures must hosted on a site that is using the 'HTTPS' protocol. URLs using the 'HTTP' protocol will not be allowed. You need to use the 'HTTPS' protocol for the picture URLs.
Making your listings more secure: https://www.ebay.com/help/selling/listings/making-listings-secure?id=4363