WP Fluid Images is a WordPress plugin by ramseyp that replaces the fixed width and height attributes so your images resize in a fluid or responsive design.
WP Fluid Images runs when you insert an image into a Post or Page. It removes the fixed width and height attributes from the image tag. If you insert an image from the image uploader, by default, a width and height attribute is inserted into the image tag with fixed pixel values. This plugin prevents this from happening, because this can be problematic if your theme is built using responsive design methods. The plugin also adds a style tag in the header that sets a max-width rule of 100%. This insures that any image in a Post or Page won’t extend past the width of the Post or Page.
If you resize an image in the Visual Editor, a new width and height attribute get added to the image. WP Fluid Images loads a jQuery script that examines the .post, or .page divs for image tags. It calculate the image’s width as a percentage of the .post or .page div, removes the width and height attributes, and adds a style attribute with a width value set to the calculated percentage.