Add slideshows to your site using the jQuery cycle plugin. Pick from several different transitions, and organize slides into multiple slideshows. Add them to your posts or pages with a shortcode, function or widget.
Generate new thumbnails when new image sizes are added to your WordPress theme. You can regenerate all of your thumbnails at once, or chose just a few, or even just one image using the Media Library.
Swap out images and assets you have already uploaded without having to delete images and replace them manually. Gives you access to a new option in your Media Library entitled "Replace Media."
Picturefill.WP provides an easy way to add responsive images to your WordPress site, using the Picturefill polyfill developed by Scott Jehl and automatic image sizing.
The swiss army knife of plugins, the Jetpack plugin comes with a few dozen features to enhance your WordPress site, from carousels and comments to a full JSON REST API.