On a dataset's Primer page and in the Catalog, you are able to set featured content. For external resources, you can manually configure the preview image for that content.
However, Socrata assets will have either a standard icon or a preview image generated from the asset. Any derived view that is a chart, calendar, map, or data lens will generate a snapshot image taken from the view. In some instances, the snapshot service may fail to generate a preview image, in which case a generic icon will be used in its place. An update to the view will trigger the image capture process and should generate a new preview image.
For all other assets, including filtered views, grouped table, forms and new visualizations, a default image will be used in the featured content.