An important concept to remember is that microstock or royalty-free are stock photos and it is not necessarily the art of photography. If you are looking to make good earnings artistic images are not probably the best way, eventhough they might be accepted and sell in limited quantities. Also you might seen many photos getting lots of likes, comments praise etc in facebook, flickr etc., they usually looks pretty. But all those photos may not be suitable for stock images, since they usually lack meaning or concept.
First thing that attracts a buyer is the beauty of an image viewing in its small thumbnail size. There are some images that looks good in its original size, but losing its original impact in its reduced size. It may be due to lack of enough lighting or good color compositions.
What kind of images are better to avoid?
Travel - Travel photos may not be always in demand, except for travel magazines, travel related websites etc. In fact snaps of you rejoicing in a beach may not sell easily.
Sunsets & Clouds - They are having less sales potential.
Flowers - They are everywhere around us, so they are not hard to get it.
Pets - Common images of pets may not be in demand, unless they are of different variety or having anything special in it.
It doesn't mean that you cannot sell the above said photos, but I'm only mentioning how to save time and efforts by concentrating more on most demanding and easy to sell photos.
The major things you have to consider while taking the photos are
1. Commercial value - Microstock agencies wants images that they can sell, and not just any old images that looks good. So make sure the images you submit have a clear message, concept, purpose or use and are the kind of images that a designer could incorporate into a design or use to illustrate a concept.
Some of the demanding categories are a) medical, 2) food and drink 3) people 4) Emotions 5) Wildlife, 6) Business 7) Industry 8) Sports 9) Isolate objects
2) Execution - It means how well you took the photo, your skills to express the mood of the subject or exposing a concept, lighting, choice of local point to add emphasis and reduction of noise on the images.
3) Keywording images - This is how a buyer can find out your photo among millions of other photos. So good keywording is most important to your success in stock photography. So make sure that you don't miss any keywords that are related to your image, and its better not to exceed 50 keywords. Think about adding keywords that describe the main object, colors, concepts, feelings, lighting, people, proper names for plants & animals, location, etc. Don't use any biological names or names that people not usually used for a common subject, always use the common names. Never use any brand names. In most cases less keywords are good (within 15 words) unless more other keywords scattered your images in inappropriate searches.
Below are some popular images from photostockworld.com