For a asset having a video it is possible to generate a thumbnail from a specified position in the stream. The image can be generated in advance or when needed depending on the use case. Thumbnails are served through a CDN so they will just be generated when not yet available. When generating it is also possible to provide a resolution, width and height.
Asset thumbnails is part of the Asset section in exposure and the time can provided either as a time in the video or as a wall clock time (requires that the video is ingested with wall clock time).
When exposure is called it will not actually return the image, it will instead a return a http 307 redirect with an url pointing to the image.
Endpoint documentation can be found here
Note that the asset ID can be provided either using the real asset ID or using the external asset ID that can be set by the customer. (Using external ID requires a feature activation done by Managed OTT support, its not enabled by default)