The parameters 'largeThumbnail' and 'screenshots' are part of ArcGIS Marketplace subscription, which can only be accessed by approved provider aka a business partner of ESRI. Please refer the following information:
Once we get access to ArcGIS Marketplace, we would be able to access the 'Listings' option under your application or services, where we can upload branding images(largeThumbnails) and Screen captures(screenshots).
largeThumbnail - Upload up to two PNG, GIF, or JPG thumbnail images of your listing—a large thumbnail (286 x 190) and a small thumbnail (200 x 133). The large thumbnail appears as your ad image in search results and the small thumbnail appears as your item icon on the details view and item page.
screenshots - Upload up to four PNG, GIF, or JPG images for the overview page, with 1 MB max file size (per image). Allocated space (per image): 286 x 215 pixels.
This is also documented in the following web resource.
Create and publish listings:
Become a provider: