How to Use Featured Images
If your theme isn’t compatible with featured images, you need to first set it up. You can Google “WordPress Featured Image Theme Setup” and find plenty of resources, or call Flash Buddy and have him do it for you. Make a note of the particular size you need. Yes, you can have more than one size featured image.
If your forging ahead with a compatible WordPress theme (all mine are), and if you are using WordPress 3.x, you should see a Featured Image box on the right-hand side of the post or page edit screen. Look for the link to “Set Featured Image”. If you are using WordPress 2.9 it will refer to it as a Post Thumbnail.
Clicking the link will open up the usual WordPress image uploader where you will upload the image as usual. It is important here to confirm the image you are uploading is large enough for its intended use. Make sure that it is either the same size or larger than what the final image will be. Once this is completed, click the link that says “Set Featured Image”, which is to the right of the button that would otherwise insert it into the post or page.
When the aforementioned steps have been completed, a preview of the image will appear in the right-hand Featured Image box.
Note: WordPress will select the appropriate image size for the location it is targeted to. The resizing and cropping are setup in your themes functions.php file and the image placement locations are configured in your themes index.php, home.php, page.php, post.php, single.php, or any customized template files.