How to Do it Yourself
You’ll need to edit a file on the server where your WordPress is installed. It’s just one line of code. The hardest part is getting access to the file called functions.php. WordPress offers a couple ways of doing this. One way is simple in theory so it naturally doesn’t work with my WordPress installation; the other involves a couple extra steps but it has never let me down.
From Inside WordPress
Along the lefthand navbar on the WordPress dashboard, pull down the Appearance menu and select Editor. The “Edit Themes” panel will appear.
Along the righthand navbar will be listed all the files associated with your theme (see illustration). Don’t freak out if the files listed on your theme are different; each theme has a lot of customized files. But functions.php is one all themes must have.
Click on the link to the file. You’ll see the code appear in the big window in the center of the page. You’ll be able to add the new code in that window.
WATCH OUT for a warning below the edit window that says: You need to make this file writable before you can save your changes. See the Codex for more information.
If you don’t know how to change your file’s permissions, and the advice the Codex gives you makes you want to tear your hair out, don’t worry. There’s another way. Skip ahead to the next section.
If you can save your changes on this panel, copy the following line of code:
add_theme_support( 'post-thumbnails' );
Paste the code on its own line right after the <?php at the beginning of the file, like so:
<?php
add_theme_support( 'post-thumbnails' );
Click Save; you’re done with the hard part. Skip ahead to the Using Post Thumbnails section of this article.
