adamprohack

<h3>What does this plugin do?</h3>
This plugin generate image thumbnails only when site need it, when someone call function with needed size, 
and saves disk space when image is uploaded to media library. 
You don’t need to use <code>add_image_size()</code> function.
<h3>How it works?</h3>
<ol>
<li>Upload image to media library</li>
<li>Change function <code>wp_get_attachment_image_url()</code> to <code>get_otf_image_url()</code> in your wordpress template</li>
<li>When function is invoke the first time, the image is dynamically adjust, create and stored</li>
<li>Page serve image in need size</li>
</ol>
<h3>Documentation</h3>
<pre><code>`php
function get_otf_image_url(
 $image_id, $size_name, 
 $width = 0, $height = 0, 
 $crop_mode = ["center", "center"], 
 $mime_type = "image/jpeg", $quality = 70)
`
</code></pre>
Parameters
<ul>
<li>Required: 
*$image_id – attachment id 
***$size_name** – dynamic size name 
***$width** – image width 
***$height** – image height</li>
<li>Optional 
*$crop_mode – image crop mode 
***$mime_type** – result image mime type 
***$quality** – result image quality</li>
</ul>
Return Value
<ul>
<li>Returns an url string</li>
</ul>

WP Images OTF Plugin

huiz

The plugin affects the uploaded image while it is uploaded. Straight 
after it is uploaded, it it resized physically to the given width. 
There is no original image left, nor made a backup. Also EXIF infor- 
mation is lost.
A lot of WordPress users or developers who use WordPress as a CMS 
and want to configurate it for easy use, would like to have an option 
to resize uploaded images. Some images come straight from digital cameras 
and exceed 4000 pixels in width. So it would be nice to reduce that to 
the max width you use inside your WordPress theme.
If you want to keep control about perfect sharp and resized images, you 
better use photo editing tools on your computer, not a script like this.
The plugin uses a class originally from Jacob Wyke (www.redvodkajelly.com).

Resize at Upload

This plugin resizes uploaded images to a given width.

WP Images OTF Plugin vs Resize at Upload