What is the difference between set_post_thumbnail_size and add_image_size? Is set_post_thumbnail_size for all images, and add_image_size for sizing images individually?
When an image is uploaded in WordPress, it is saved in its original size, and also as some resized copies in different sizes. In this way it becomes easy to use different sizes, of the same image, for different purposes.
By default, WordPress creates 3 copies in 3 different sizes:
The size in pixels for these 3 sizes can be set in the WordPress dashboard, under the menu
Settings -> Media.
Now, the function
add_image_size is used to register a new size, that adds to the 3 defaults.
All these functions accept a parameter
$size that should be the name of one of the registered sizes (one of the 3 standards, or one of the custom sizes registered with
'full' is used as
$size argument, the original image is returned, the one not resized.
These two functions get (the first) and echo (the second) the image that is set as "Featured Image" for a post.
What is returned (or echoed) by these functions is a full
img html tag, something like
<img scr="http://www.example.com/wp-content/2013/08/image-200x200.jpg" />
So, which is the size used?
$size is passed as 2nd argument (1st is
$postid), then these functions return the image in that size. Otherwise, these functions search for a image size registered with the name:
This is not one of the 3 default sizes, in fact, it is the 4th standard size (the 5th is considered
full) and you can register it calling
set_post_thumbnail_size( $width, $height, $crop );
is a shortcut for
add_image_size( 'post-thumbnail', $width, $height, $crop );
If you haven't called
set_post_thumbnail_size (which means the size
'post-thumbnail' is not created), WordPress will use the size
thumb, and if even this size is not available, WordPress will use the original image, the one not resized.
Registering custom image sizes can be very helpful when designing a theme, but it's important not to abuse: every uploaded image is copied and resized for all the sizes registered, so registering a lot of sizes will heavily slow down the image uploading process.
Be aware that when WordPress creates scaled copies of an image, it never enlarges it, but only makes smaller copies: e.g. if the original image is 400x500px, and the 'medium' image size is 800x600px, medium size and larger are not created.
So registering an image size never gives the security that an image file for every size registered actually exists in the WordPress content folder. Even because registered image sizes easily change: changing settings, switching themes, etc.
When an image size changes, for any reason, the change has an effect on the images uploaded after that change; images uploaded before don't change dimensions and are not rescaled and resaved.
If one needs to rely on specific image sizes after some images are already uploaded (e.g. after changing themes), then the plugin Regenerate Thumbnails will be a life saver.
Set the default Featured Image (formerly Post Thumbnail) dimensions. To register additional image sizes for Featured Images use: add_image_size().
Registers a new image size. This means that WordPress will create a copy of the Featured Image (formerly known as post thumbnail) with the specified dimensions when you upload a new image.