Yes they are! Great question. I wrote a hub about this: Get Hits on Hubpages - How to Use Keywords in all the Right Places.
The subtitles in for all capsules (texts, photos, polls, tables etc.) are coded as H2 heading tags in HTML. And photo captions are coded in the IMG ALT tag/attribution in HTML.
Search engines pick up all this html and the image captions can also help a Google image search find your images - sort of a back door into your hub. For a more thorough answer you can read the hub since it is too much to write here. Good luck!