If I have amateur astrophotography image data that might be of scientific value, what should I do? I have some image data that may be interesting to someone, somewhere, but I don't know who to contact or notify.

I would imagine it would depend on the type of data, but maybe there is a central place people can contact?
It depends, as you say, on the subject matter. For instance, images useful in variable star study can be provided to the AAVSO http://www.aavso.org/ and images relating to asteroids can be provided to the IAU Minor Planet Center. http://www.minorplanetcenter.net/ Other subject matter will be accepted elsewhere.

However, for the images to be useful in scientific fields, you will need to comply with the submission instructions, which a casual snap probably will not comply with. On the MPC guide for beginners there is a list of 44 technical suggestions for submitting scientific data: http://www.minorplanetcenter.net/iau/info/Astrometry.html The AAVSO has a tutorial in six chapters on photometry using a DSLR camera (and another for astro CCD cameras) http://www.citizensky.org/content/dslr-documentation-and-reduction

If you just want to get it out there and have people comment on what is in it, post it to Google+.