Smush is one of the best plugin in WordPress. It is important to understand Smush is an optimization tool that uses lossless image compression to save disk storage space and speed up your site. Smush squeezes data from your image files that will save space, resize huge images, and improve speed without changing quality.
This plugin come into free version and Pro version.In free version there are so limitation such as you can only 50 images optimize at one time.