I don't think they are cavities.
For the most part, you don't have to worry a lot about dog's teeth. Her teeth look really clean.
Dogs that are fed only kibble tend to have pretty decent teeth. What some people do (I am one of them) is to give their dogs bones ocassionally - real raw bones (just ask at a meat shop) - big ones, joints, that they cannot possibly break apart with their teeth (and since they're raw they shouldn't be a problem even if they did). Some people aren't comfortable with this because they feel it is a risk to the dog. If you do give her a bone, watch her while she is chewing it to be sure she isn't breaking it into pieces. I have yet to see a dog break one of these up - and my german shepherd gives it a good try.
What kind of dog is she?? (Just out of curiosity)