Alexandru Vornicescu

This is a WordPress plugin.
If you want to resize images in your posts automatically so the page will load faster and use less bandwidth then this plugin can help you.
To take effect just open a post or page and all images in that post will be resized to a smaller version.
It will replace the URLs of the images that are too big with URLs of the resized images that will fit into the posts.
The plugin will keep the original copy of all image files.
Note: With version 4.3 now you can resize images in Text Widgets.
Note: The filter – “wp_make_content_images_responsive” will be deactivated because with this plugin we will not need anymore.

Resize Images In Posts

This plugin will resize images in your posts.

jegbagus

This plugin have ability to override default worpdress behavior and disable croping image but try to resize image to fit rectangle container size and fill space with background color. 
You also able to choose background color to use.
Links: <a href="http://farm9.staticflickr.com/8459/8016053956_8616f8b1f8_z.jpg" rel="nofollow ugc">Example</a> | <a href="http://jegtheme.com" rel="nofollow ugc">Author Homepage</a>

JResizr

Resize with no crop and fill background color of the rest area

Resize Images In Posts vs JResizr