There are a few routes, but it's very competitive, and extremely time consuming. You need the right gear (if digital, an DSLR, because of resolution). There are stock agencies, like Marine Themes, and Painet. You need tons of images, and they keep 50% of everything they sell. You often have to pay for dupes (from slides) or have hi-res scans made. You will not get rich. 
You can approach magazines, but again competitive. Ask to be put on their needs lists, and you might just have what they're looking for.
You could have your own web page, as long as it has a searchable data base.
You may also want to check a resource which comes out every year, Photographer's Market, which lists agencies and magazines that accept free-lance work.
Good luck,
Marli