You can add up to 24 picture URLs in one listing (for almost any category at no cost)and for multiple-variation listings, a maximum of 12 pictures may be used per variation.But you cannot mix self-hosted and EPS-hosted URLs in the same listing. This can be done via the UploadSiteHostedPictures API call to upload pictures to EPS. Specify your image URL in the PictureURL tag of the AddItem/AddFixedPriceItem calls.
To specify multiple pictures, send each URL in a separate, PictureDetails.PictureURL element. The first URL passed in defaults to be the 'gallery image', and will be the first picture that is shown when a user lands on the View Item page.
Note: If pictures are externally-hosted, they must be hosted on a site that is using the 'https' protocol. URLs using the 'http' protocol will not be allowed.