Can I legally sell photographs of animals (dolphins, otters, killer
whales) that have been taken at a marine park? There are no
indications that they were taken there (i.e Sea World logos)  Many of
these pictures were taken behind the scenes and I think are hard to
come by.  Do most companies shy away from purchasing these?

http://milov.nl/forum/4/799

check out here

you can "sell" your photos online, http://www.istockpro.com/index.php
is a site where they post your work and sell it for you

its where independant photographers can make a little bit of cash,
selling their work as "royalty free" images to be used by whom ever
purchases it.