Generally, photos are owned by the photographer that took them, unless they sign away their ownership to the image (temporarily or permanently).
Not "generally." Always. The photographer owns the picture unless he has agreed otherwise. Even if someone else bought the film.
Notice, please that while many photographers specifically assert their copyright often by adding a copyright notice to the picture, you don't have to specifically assert a copyright to still own one. So, just because a picture doesn't say "copyright" on it, doesn't mean that it isn't.
A knife maker may have a copyright on the design of his knife (as an artist, he does. Copyright applies not just to printed text and pictures, but to just about any creative product). But if I take a picture of it, then I hold the copyright for that specific picture of that knife. I can even make money by selling copies of my picture of the maker's knife even though the knife itself is copyrighted by the maker.
There is the issue of "fair use." Fair use says that you can, within certain limits (do a websearch for the details) copy something from a work for the purposes of discussing that work. A book reviewer, for example, may quote a paragraph or two from a book he's reviewing. Fair use says that if the picture is part of a broader work, an advertisement, for example, and you want to discuss that broader work, you can "quote" the photograph. So, if I want to discuss the latest Benchmade knife, I can quote a picture from BM's website (though you must acknowledge the source and note that it is a copyrighted work).
Fair use also allows you to copy a copyrighted work for your personal use. If you're doing some research on a subject, for example, and you go to the library and read some books, you can photocopy pages out of those copyrighted books for your own use. But you can't republish those pages (there are actually ways you can republish them. Again, do a websearch if you really want the gorry details.)
Let's take the case of a customer who buys a custom knife from a certain maker. Before delivering the knife, the maker takes some pictures of it. After paying, the customer obviously owns the knife. But, the maker still owns the pictures he took of it. The maker can, for example, include those pictures in a printed catalog or brochure or on his website or exhibit those pictures at shows, etc., even though he doesn't own the knife. On the other hand, the customer does NOT own those pictures. So, he can NOT take a copy of the picture off of the makers website and post it on his own website. He CAN, however, take a copy of the picture off the maker's site for his own personal use. For example, he can put that picture in his own private files or collection catalog, use it for insurance purposes, etc.
Could he post it here on bf.c? Msybe, if his purpose is to discuss the knife and if the picture does not constitute the majority of his post. So, to just post saying "Look at my new knife." Well, that's not really discussion, is it? The majority of your work, your post, is the picture. To qualify as fair use, you have to actually contribute something to the new work, the post. You could give some of your own thoughts or opinions or comments about the knife, for example.
Let's discuss one more little issue. Let's go back to the case of the maker who sold the knife. Even though he sold the knife, the maker still owns the copyright to the knife itself! So, the owner can't take the knife to another maker and say, "Make me another one just like this." The second maker would be violating the first maker's copyright and the customer would be engaging in a conspiracy to do so.
If the original maker wants to, he CAN make a second knife just like the first and sell it to someone else. He owns the copyright. When you buy a custom knife, you can ask the maker to agree voluntarily not to ever make any more. That's fine. But, the maker still owns the copyright even though he's agreed not to every reuse it.
Let's say that you own a custom knife that you like very much. For you, it's perfect. But, it's getting a bit old and worn out. The maker is retired. You know another maker who does very similar work that you also admire. Can you take him your perfect knife and say, "Make me another one just like this?" NO! Not without getting the permission of the original maker.
And what if the original maker is dead? Then you'll need the permission of his estate for something like 99 years (looking up the exact time period is left to the reader as an exercise.)