Taking nice photos and selling them are two radically different things. For 13 years I owned a nature oriented "gift/gallery" in a very beautiful scenic area. The western states, Michigan's Upper Peninsula and Maine are studded with similar stores and dedicated galleries. "Rocks and trees" sell well in those areas, but probably not so well elsewhere. (I bet reef fish sell well in Florida.) There is a lot of competition for scenic nature photos, so as we mentioned earlier, self promotion is very important. But the images had also better be spectacular because the buying public is brutal. If they look at your photo and the one next to it and they like the one next to it better, guess which one they are going to buy? And if that photo is digital, most of them will think "modern" and to most of them modern is good. If they happen to know that the lifespan of digital images is short or unknown, but the little card says "printed on professional paper using archival inks" or something like that they will breathe a sigh of relief and buy it.
There is also the cost consideration. Painters and other fine artists share similar concerns with photographers. If you paint a watercolor, you can only sell it once. But if you have prints made from it, you can sell lots of them. The original will sell for more - if someone likes it enough, and they'd better if you're going to invest in making the prints - but it probably won't sell for enough to make a good wage on the time invested. The prints will sell for less, but you can keep selling them over and over. Of course there is the question of how many to make and what quality. It's all basically manufacturing/marketing at that point. And people buy most prints as they would any other home decor item. Some artists are able to command pretty high prices for limited edition prints, but most cannot. I suspect the same is true for photographic prints, though we never carried them ourselves because a neighboring gallery specialized in them.
I suspect that the popular subject matter varies in different parts of the country, and I would expect people to be the most popular subject in big city galleries. But I suspect all the same things apply. Art collectors and aficionados will buy original paintings and the highest quality photographic prints and will pay the highest prices for the biggest names. Those thought to be good investments. Most people will buy lower quality prints from the famous artists, or originals or prints from lesser artists, will pay less for them and will have lower expectations. Except for the weird notion that a reproduction of a work of art can ever be an investment. I've just never understood how a copy of a painting can be an investment. The one thing photography has going for it is that reproduction is an intrinsic part of it. The "print" is the art. To some extent that cheapens it in the eye of the public, not to mention that they all believe that their new automatic wonder camera can take the same shot all by itself. But it also means that if you printed it, it's your original creation, not just a manufactured reproduction.
Here's something I haven't run into, though I've been out of the art retail business for a few years: Is anyone making high quality prints on fiber paper with real gelatin emulsion and selling them as artistic originals, and then scanning them and selling prints from the scans as cheaper "prints"? This would be a similar business model to the painting/print market. It might be an angle to look into...
Oh, one more thing that I think most people are not aware of. If you were to go to a wholesale gift show, you would find vast booths of production "art". They sell framed prints from original paintings or photographs and they sell them by the zillion. Most are probably manufactured in China, though some are very nicely done. They do not look for the most artistic work, they look for work that fits the current home decor styles and colors. This is really product manufacturing, not art - the artist or photographer is the "designer" or "design engineer", if you will. If you are a pure artist, you might snub your nose at that, but if you want to eat, you might think about getting some of your work into that market. Some very well known artist's work shows up there, so you'll be in good company. Most of the retail buyers will agonize over their purchase, but frankly, most of them don't really care who created the original. They just want a nice picture for their wall. If you can manage to get into that market, and keep giving them what they want, it can be a steadier income than high end gallery sales.