Primary and Secondary Resources

Firstly, a primary resource is an image that you have taken yourself, or even something that is right in front of you, like a fruit bowl, or a nude woman. A secondary resource is an image that someone else has created. Images in a concept art book, for example.

                  (An example of a secondary image)

Primary images are a great reference, since you've taken them yourself, you will be looking at exactly what you need to be creating what you desire - angle, scene, objects will all be how you want them. Even the colours and mood will all be set by yourself. All be-it, they can be tougher to get a hold of. Firstly, you'd need a camera, but more importantly you need to be able to find/create the sources you want to capture. For example, primary observation for landscapes would mean you must explore in order to find what you are looking for. For the image above, some research of stalagmites would perhaps be useful. Finding a cave full of stalagmites isn't an easy task. This is where secondary recources come in.

You can pool together a large collection of other people's images very quickly nowadays, thanks to search engines. If you need 20-30 photographs of stalagmites or stalactites in a cave, it will take a matter of minutes to amass what you require. This amount of reference is incredibly useful. However, the downside is that you didn't take them yourself, therefore they might not be to the taste of what you want. Perhaps all the photos you find are of concrete stalagmites, when you really wanted more pictures of limestone stalagmites. Maybe the angles of the photographs aren't interesting enough, and you would have liked a different persective. Or, on the contrary, they're much better than any image you could have put together yourself, and prove a higher quality reference.

A good balance of the two is perfect to create an accurate and belieavable scene,object, or creature.

                     (I don't have any way to add a primary image of my own in to here, so here's another secondary instead.)