We’ve all been there. You need a special size of image for your WordPress install. Maybe it’s a unique dimension for a slideshow. Maybe you have multiple featured posts with thumbnails section that use different sizes of thumbnails. Whatever your reason, instead of letting WordPress load the larger image and downsize it on the fly you can use the “new” WordPress plugin “Addtional Image Sizes (zui)” to create the new image sizes and make copies of and resize all the images you’ve already uploaded so they all have the new size.
Get the source code for Additional Image Sizes (zui) at github. Or search WordPress.org for “additional image sizes zui” (coming soon).
This functionality is also included as a ready to use appliance in my “platform/framework” WordPress plugin Kitchen Sink HTML5 Base. You can check out the source code of the KST Kitchen Sink HTML5 Base WordPress plugin at github and read the fairly extensive wiki documentation there as well. Or you could visit the “Twenty Eleven” demo theme for Kitchen Sink HTML5 Base where all the slideshows in the demo use a custom sized image created with this plugin.