Maybe because of my cliche that taking photographs is easier. It lasts just a moment and you are done. Getting involved in stock sale shows that it is far from easy job. With my photographic skills I should mix a bit of luck in the recipe too. Anyhow, I sold some of my photos. Pictures and scenes all around us are perfect, proportional, colorful but it takes skills to place them in a frame with all the requirements. When you get equipment and some knowledge you are able to take technically correct images. But it is not only image that sells, it is overall idea as very important part.
After you get right idea then you face another obstacles in executing it. Firstly, the idea is born in your mind, so it may be difficult to find the same composition to shot. If you can arrange it it asks for more efforts, including model releases and all other stuff. I have plenty of ideas but was unable to make a photo for mentioned reasons.
There are techniques that one could understand and learn to successfully make this kind of art. It helps to read tutorials, to look some others work. One of the most valuable advantages of illustration is freedom to get an ideas which cannot be executed with camera. In this article you can see some of them. Illustration releases imagination to full extend. Sure, there are more simple illustrations where excellent idea can prevail the artistic point of view.
But making almost hyper realistic pieces of art is what really amazes me. Just look at Poddubnygennady's portfolio and you will see some great illustrations.
At the moment I see that my interest goes in 3 directions.
Outlining photos - Pretty nice for beginning. Search for edges and outline them in vector based program, add some color and maybe artistic touch. It is important to take care of copyright regulations for the images you use. On many photos it is prohibited to do such an derivative work. Fortunately there are plenty of Public domain images over the internet, which you can use freely. And you can use your own photos that has good composition and idea but are technically incorrect. Why not make illustration out of them.
Standalone illustration from scratch - It raises from an idea. You can imagine whatever you want and draw it from scratch. Concepts, metaphors, messages etc. sells well, IMHO. Making illustration from scratch requires your most skills because you don't have any shape or colour template to refer to. And you can illustrate impossible scenes like man walking a dog on the moon. By making illustrations from scratch you can benefit from your researches on download ratio for some keywords. Some abstract terms like success, failure, depression (and all other emotions) can be expressed by different metaphors or concepts. Good metaphors, I believe sells well. Conceptual art, specially in business area is wanted too.
There are also other techniques like scanned hand drawing etc. Maybe I will post some illustration tutorial later.
Illustration consumes time but I think it is worth once you get your own style.
And at last, don't forget that search engine is a bus that drives to your station, but only if you put final effort to Title, Description and Keywords.