An Avatar is an image that represents a person or company. Clicking an avatar typically takes the user to the corresponding profile page. A placeholder avatar is used when an image has not been uploaded.
Thumbnails can be used for images, videos, maps, and documents. Clicking an image or video thumbnail opens it in the Media Light Box.
When to Use
Use avatars to provide a human face to the cold, hard text it sits next to. Examples: profiles, chats, lists of humans, etc.
Use thumbnails of photos to provide a preview of the item it links to. Examples include: training videos, photos uploaded to an invoice, receipts uploaded to a sales transaction, and items attached to a note.