Let’s kick off this list with WP Smush Pro from our own stable of premium plugins.
WP Smush Pro lets you compress JPEG, GIF and PNG images individually or in bulk using our dedicated smushing servers.
The free version of this plugin, WP Smush.it lets you compress images up to 1MB in size, while the pro version smushes images up to 5MB in size.
Both plugins work by stripping meta data from JPEG files, optimizing JPEG compression, converting certain GIFs to indexed PNGs and stripping the un-used colours from indexed images.
Site performance tool GTmetrix.com also recommends WP Smush.it as one of theiroptimization tips for WordPress sites.
EWWW Image Optimizer
EWWW Image Optimizer automatically and losslessly optimizes your images as you upload them to your site. It can also optimize images that you have previously uploaded.
Another nice feature of this plugin is the ability to convert your images to the file format that produces the smallest image size. It can also optionally apply lossy reductions for PNG images.
CW Image Optimizer
CW Image Optimizer is another plugin that automatically and losslessly optimizes your images as you upload them to your site, and can optimize images previously uploaded.
This plugin is based on WP Smush.it, though unlike the WPMU DEV plugin which uses the Yahoo! Inc. Smush.it service, CW Image Optimizer uses the Linux littleutils image optimization tools. This means your images never leave your server.
Installing littleutils can be fiddly so this plugin it’s for noobs – it’s easier to install any of the other standalone plugins in this list.
Don’t be scared off by Imsanity’s freaky feature image in the WordPress Plugin Repository. This free plugin automatically resizes images to a more “sane” size.
A nice feature of Imsanity is the ability to set a maximum width, height and quality. You can also convert BMP files to JPG so images can be scaled.
If you take adaptive design seriously, you might want to consider using Hammy. This plugin takes your images and regenerates a number of smaller sized images. So when a visitors arrives on yours site, they see the most appropriate sized image (or the smallest one possible). This makes for a better and faster experience, especially for mobile users.
Hammy only works for images within posts and pages, not custom post types.
PB Responsive Images
PB Responsive Images automatically reformats all your images into a format similar to the picture tag proposed by the Responsive Images Community Group on w3.org.
Each image is reformatted based on standard CSS media queries and SLIR query pairs, giving more flexibility in the variety of query combinations. Also, the plugin provides shortcodes so you can customize the queries used per image, and helper functions that can be used to customize the queries used per post or per layout.
SEO Friendly Images
SEO Friendly Images edits all your images with proper ALT and TITLE attributes to help boost your SEO. If your images don’t have ALT and TITLE already set, this plugins adds them in according to the options you set.
Is it worth using ALT and TITLE attributes? Well, yes. The ALT attribute describes your images to search engines, while TITLE automatically appears in the tooltip when a user hovers over an image.
Media File Renamer
Media File Renamer allows you to rename your media files by updating their titles. It also automatically updates their links in posts.
Search engines like Google give a lot of importance to file names so it’s important to be descriptive and use appropriate keywords to improve the SEO of your images.
Let’s not leave lazy load plugins off this list. Lazy load doesn’t need configuring because it works out-of-the-box.
It uses jQuery.sonar to only load an image when it’s visible above fold. This plugin is an amalgamation of code written by the WordPress.com VIP team at Automattic, the TechCrunch 2011 Redesign team, and Jake Goldman from WordPress consultancy 10up.
BJ Lazy Load
BJ Lazy Load allows you to lazy load selected images, including post images and thumbnails, Gravatar images and iFrames, and replace content with a placeholder.
Unlike Lazy Load, there are some simple settings that allow you to customize how the plugin works, such as choosing a placeholder and skipping images with classes.
This plugin caters to size optimized images, automatically serves scaled down images in responsive designs, and automatically serving hiDPI images for hiDPI screens (like Apple’s retina display).