In previous versions of WordPress (prior to 2.9 if I remember right) there wasn’t in-built possibility to use post thumbnails. That’s why some themes (the best of them) used custom fields to hold a related image URI. Some of themes retrieved the first image from posts. Each theme had its own method to store/get the image and that’s why changing a theme was a quite time-taking process.

It seems now that was ages ago. Currently the most of the WordPress themes support in-built WordPress functionality of post thumbnails. But probably it’s not obvious how to set them because I receive questions of them every day.

Firstly,

What are the post thumbnails?

A post thumbnail is a small picture that displays near the post excerpt on pages like index page, category and other pages where a list of post appears.

How to set a thumbnail for a Post?

Generally there are 4 steps to do this:

1. Create a post
2. Upload an image to your blog
3. Set the image as featured image
4. Save the post

Create the post

Ok, it looks obvious and I will not describe the step :) Just note that you can use any of your old posts for that.

Upload an image to your blog

Now when you are on the add/edit post page there is the link “Set featured image” in the right sidebar. Just click the link:

You’ll be taken to the “Set featured image” window:

As you see the window is exactly the same as any other “Add Media” window. So you can upload an image there. Or you can use any of previously uploaded images from Media Library.

Set the image as featured image

After you’ve uploaded an image, click “Show”. At the very bottom of the image properties you’ll see the the link “Use as featured image”. Click it:

After you click the “Use as featured image” link, you’ll see that the featured image is set for the post:

Save the post

Nothing new here. Just save the post.

That’s all. The post thumbnail is set!