Greg - SiteOrigin

A lot of themes ship with Timthumb because it allows their users to start using the theme without rebuilding thumbnails.
Secure Image Resizer offers a far more secure alternative. It allows theme developers to rely on WordPress’ own image resizing functionality using <code>add_image_size</code> and <code>get_the_post_thumbnail</code>.
When get_the_post_thumbnail is called, Secure Image Resizer checks if the requested image size exists. If the image doesn’t exist it dynamically creates the resized version. It works in the background, with no front end functionality.
Theme developers can either bundle this plugin into their themes, or suggest that their users install it.
It also includes 2 functions <code>so_resize_custom</code> and <code>so_resize_custom_html</code> to do dynamic image resizing using Imagick. Great for theme developers.
Please Note: This is an early release of Secure Image Resizer. Please use it, try and break it and report any bugs to me support@siteorigin.com so I can quickly fix them and work towards version 1.0.

Secure Image Resizer

A very simple and secure image resizer. It adds dynamic resizing so you don't need to regenerate thumbnails when you install a new theme.

mores

This plugin is basically just a little improvement of <a href="https://wordpress.org/extend/plugins/resize-at-upload/" title="Resize-At-Upload" rel="ugc">Resize-At-Upload</a>, a fabulous plugin by A. Huizinga.
The plugin will automatically resize an image upon upload, depending on the maximum width and height that you define. Gone are the days when you, or your client, will ruin a site’s layout by uploading a huge file with 25 megapixels. 
Be advised: there is no backup, no copy of the originally uploaded image.
The plugin uses a class originally from Jacob Wyke (www.redvodkajelly.com).

Resize at Upload Plus

Automatically resize (i.e. scale down) images after uploading. Save disk space and preserve your layout

Secure Image Resizer vs Resize at Upload Plus