Using photos and drawings from the internet. Copyright,credits and what is public domain?

There is an important question that concerns all bloggers. How to illustrate the articles that they write?

The article could need the picture of an animal, machine, family, girl, old man, cute baby, lush green spinach, yummy looking chocolate cake or some such.
The obvious way is to click a photo using one's own camera and use it.
Or go through one's collection of old and new photos and choose the one that best illustrates the text in the blog.
The creative ones draw their own line figures, drawings, or cartoons.
So far so good.
What if you don't have the photo or drawing you need and also don't have the means to get it quickly and easily?
Simple, you just search the internet ocean with the key word of the image required and presto there are many that match your needs.

Well, can you and would you just copy and paste the ones you like best and that is that?
I know one is tempted to do this as the easiest option available.
Here is the catch: All the photos, images, drawings and illustrations that you get by any online search are owned by the people who originally clicked them or created them. (I know you already know this in your heart.)

Well, some of the artists may have generously donated their creations to the world and you can use these for free giving due credit to the original creator.
Most others are copyrighted  material and it is illegal to use them in your blog, article or elsewhere.

Who is to catch me, you say.
 Well, no one may catch you given the vastness of the online world but you are stealing all the same.And you can be caught too if someone tries to.
This is plagiarism. Plain and simple.

So, are there any pictures or illustrations that you can use in your content without worrying about propriety or legality?
Yes. The images that belong in the PUBLIC DOMAIN can be used by all and no permission is required for these.
What is public domain?
No, these are not images that you see in public on Facebook, Twitter or personal blogs.
 All of those are copyrighted material too and you cannot use them unless you ask for and get the author's permission.
Images in public domain are those that have been used and seen so often that they are not restricted for use any longer. For example, a picture of Shakespeare, Einstein or Mahatma Gandhi that has appeared and re-appeared in countless places. I know the examples are not adequate but you can research and find images that are thus available.
For all other images, you have to ask for permission before you use them.
You may get the permission for free with or without the rider that you will give due credit for the image to the owner.
Even if the author does not ask for a credit line it is common courtesy to give due credit as mentioned.

Or you may be asked to pay for the right to use the image and the price could be any amount depending on its perceived value.

This is the right way, the professional way as I have learnt working in the publishing industry.

Please don't use others' images without going through due process of permissions and credits.

Update: I was thinking where one could find some images for use. I researched the internet, like all of you do I am sure and found something which will really be helpful to those looking for images that they can use without breaking any laws. This is a comment under an article. Click on the link given as 'here' and you will see thousands of images on Flickr that you can actually use if you don't have any of your own.

Pete Stevens says:

You can freely use images from flikr that are under the creative commons attribution licence. Just add a credit for the photo.

You can search the Flikr creative commons here