Suppose I made a commentary video about SpongeBob. Would using an image of SpongeBob in the thumbnail be considered fair use in this case?
Most likely, as it seems to meet the standards of:
- Relationship to the whole (small part of the animation)
- Effect on value of the copyrighted word (likely negligible effect)
- Purpose and character - commentary, uniquely identifying the subject of the commentary.
The outstanding portion to be considered, nature of the copyrighted work, as a work of fiction targeted at children, is not in your favor (you aren't trying to copy a phonebook, for example), but given the other factors, should not weigh heavily enough against you to cause problems.
That said, fair use might not be enough to protect against a copyright bot, so modifying the image to show context of this as a commentary would probably be beneficial in more than one way.