Something like this:

add_image_size( ‘sidebarimage’, 500, 400, true );

In the string, you give this custom size a name. I’ve called it “sidebarimage”. Then I’ve set the WIDTH and the HEIGHT. Finally, I’ve told this function to automatically crop the image, instead of resizing it dependant on the width or the height. You can not set this, or set it false, and it will resize the image down to the either the maximum width or maximum height… Sometimes it’s useful to set this so the height is 9999 so you can have a flexible height, yet a fixed set width. Up to you how you roll with that one!

You call back the function like you normally would, instead you call back the ‘sidebarimage’ inside the brackets of the_post_thumbnail template tag, like so:

the_post_thumbnail( ‘sidebarimage’ );

You can set up multiple add_image_sizes with different callback names and call it back wherever on the website. Amazing stuff!

More info here: http://codex.wordpress.org/Function_Reference/add_image_size

I love the WP tips, helps out a ton.

.mike