This plugin behaves similarly to Regenerate Thumbnails except that images are resized automatically / on the fly, when they are used. Once created, they won't be processed again.

Your thumbnails will now resize when:

• Image Sizes in Settings > Media is modified,
• Switching themes & plugins with different thumbnail / featured image sizes

No settings pages, just install and activate and things should work right away.

Get it from the WordPress.org plugin page

Test it out. In your normal WordPress website set up, create a gallery using the Add Media button while editing a post or page and use thumbnails. Afterwards, check out your gallery.

Notice the size of your thumbnails, most likely they're 150 x 150. Do you see it? Great.

Now head over to Settings > Media and change your thumbnail size to something cooler, something rectangular, let's try 400 x 200. Save it.

Go back to your gallery that you previously created and refresh your browser. Most likely you won't be seeing 400 x 200 thumbnails there.

OTF Regenerate Thumbnails fixes this for you.

WordPress only generates thumbnails when they're uploaded, that means that themes & plugins that have different sized thumbnails won't have the right dimensions for images that already exist in your site. WordPress uses the closest size that it already has generated instead.

There are regenerate thumbnail plugins already existing for WordPress, but those are manually triggered and goes through all of your attachments (or lets you choose from them). That may take a while especially for large sites.

Lastly, WP functions allow 2-item array sizes to be used for thumbnails; however those do not work as expected because of the resize behavior of WordPress.

This plugin solves all of the above. No need to regenerate anything anymore since they're done on the fly.

OTF Regenerate Thumbnails should work right away, and your images should get resized when the dimensions get changed.

For developers & tinkerers, ensuring your images get resized properly requires you to use WordPress' image functions to display featured images and image attachments. Don't worry, this is a good thing.

Make sure you use these WordPress functions every time you display images:

• wp_get_attachment_image_src
• wp_get_attachment_image
• the_post_thumbnail
• get_the_post_thumbnail

You can also add size names using add_image_size

• Creates Resizes thumbnails on the fly
• Handles Image Size settings changes in Settings > Media
• Handles thumbnail / image size changes introduced by switching themes & plugins
• Works automatically, no setup needed
• All calls to the_post_thumbnail and other thumbnail functions are handled automatically
• Handles image sizes created from add_image_size,
• Handles 2-item array sizes
• No custom scripts that perform saving/resizing, everything is performed by WordPress.
• Works great with Jetpack's Photon module

1. Head over to Plugins > Add New in the admin
2. Search for "OFT Regenerate Thumbnails"
3. Install & activate the plugin

This plugin deprecates our old bfi_thumb script.

bfi_thumb was also an on-the-fly image resizer for WordPress, but it included features like modifying grayscale, opacity, resizing image up, etc. Practically speaking, we rarely used those other features. And I'm guessing in a few years filters will be purely CSS also. The script became buggy and most likely it's because of those features. The script itself definitely was not created the WordPress Way.

So now, we've removed all of those unused features and now the script is a plain good ol' image resizer that uses WordPress' hooks to work, no fancy function calls that you'll need to call (ala-timthumb & ala-bfi_thumb).

Inspired by:

• https://wordpress.stackexchange.com/questions/53344/how-to-generate-thumbnails-when-needed-only/124790#124790
• http://codex.wordpress.org/Function_Reference/get_intermediate_image_sizes