HA. featured images are funny.
They are mostly dependent on your theme selection. Each theme handles them however the theme designer thought would be good.
If you’re digging through code or documentation to figure out where and when the featured image turns up… you may come up empty searching for “featured image”.
The other name for them is “post thumbnail” – this can get confusing because, actually, they are rarely ever “thumbnail” size. Nobody ever said this stuff made sense.
There are some terrific free themes that take advantage of the featured image functionality of WordPress. One of our favourites is Adventurous by Catch-Themes – it’s a well designed theme that can be customized quite extensively with a little work.
You can use a featured image to illustrate your blog listing (or whatever other purpose you’ve devised for the POSTS in your wordpress site), to make your pages and posts more visual with big header graphics, and for post/page content sliders.
If you want to get fancy, you can even manipulate your functions file to use the featured images as backgrounds on their associated pages/posts!
Default wordpress installations allow for setting MEDIA SIZES – these will impact your featured image sizes when you upload images that are quite large in dimension… WordPress will automatically resize based on your media size settings. Sometimes you want to change those, and you need to then “regenerate” your images so they become the corrected sizes.
Suffice to say – there are lots of ways to use featured images or “post thumbnails”, so experiment with your theme … use your “inspector” tool in the browser you like best, and play with how to manipulate it!