Short answer
HubSpot's KB: for blogs outside HubSpot, an RSS email sends subscribers new posts from the feed. Featured images aren't added automatically; include them in the feed. Each send only includes posts new since the prior send, even if fewer than the maximum you set.
1. HubSpot blogs
For blogs on HubSpot, use blog subscription emails. My demo portal's blog:

2. Subscription types
See subscription types.