I'm using some custom code to make the header image a random featured image from among all the posts in a certain category. I've got that part working well, but I'd like to make it easier to create the header sized featured image. The posts in the category consist of a single image that the header is usually a portion of. What I'd like to be able to do is allow the user to easily create a featured image of the correct size from the full size image inserted in the post. I couldn't find a plugin to do this, and I'm just getting started with WordPress development, so I'm not sure where to start on something like this. Can anyone point me in the right direction as to how to go about creating a tool like this?