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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.

th23

Provides easy admin access to define maximum width and height of uploaded images. Uploads exceeding these dimensions will be resized accordingly. Only the defined maximum size will be stored on the server. This way you will be able to save space and bandwidth and serve pages faster to your visitors due to the reduced image size.
Watermark your precious images, keeping track of them in the internet. New uploads can automatically be marked or you can add/remove watermarks via the Media Library. Unmarked copies of images are kept inaccessible to the public in case you want to restore it.
<a href="https://www.youtube.com/watch?v=umfS6tGseqI" rel="nofollow ugc"></a>
Additionally th23 Upload offers some further options:
<ul>
<li>Keep original aspect ration of uploaded images, preventing automatic cropping</li>
<li>Specify a suffix for resized images to be added to names, eg upload of too large image <code>test.jpg</code> will be stored as <code>test_resized.jpg</code></li>
<li>Set quality for resized images, allowing to save further space and bandwidth</li>
<li>Select image sizes to watermark, eg to excluding thumbnails</li>
<li>Mass-add/-remove watermarks for already uploaded image attachments</li>
<li>Select location of watermark on the image and maximum width/height to cover</li>
<li>Supports GD Library and ImageMagick (Imagick)</li>
</ul>
<h4>Usage</h4>
This plugin works in the background, even for Admins and Editors of your website.
Once configured all work upon upload of new images happens during the normal process – images are resized and watermarked as per your configuration before they are added in the post / page editor.
See intro videos about functionality and configuration for an overview: <a href="https://www.youtube.com/playlist?list=PLnpYL-vo05g3JRAJiL0xG6Dqc0HxZ1DJL" rel="nofollow ugc">Visit the th23 Upload video series on Youtube</a>.
<h4>NOTE</h4>
Only handles JPG / JPEG images, as PNG (transparency) and GIF (animation) could loose their features upon resizing.!
For manually adding / removing of watermarks visit the Media Library, where you find this function as part of an images row actions, when hovering.

th23 Upload

Resize images on upload to maximum dimensions, saving space and bandwidth. Watermark images upon upload or manually via Media Library

Resize at Upload vs th23 Upload