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When you insert media with the media upload, it will generate images of several sizes according to the media settings. But sometimes I only need thumbnails. Other times I only need medium images. If each post has images that aren't being used, it waste space.

Is it possible to select custom image size to be generated when you open Insert Media?

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PS: I mean, for one post, only generate thumbnail, NO medium; for next post, generate thumbnail and medium. – user32161 Jun 8 '13 at 9:06

You can choose the sizes that WordPress automatically generates for images in your Dashboard Settings under Settings > Media, which is located at wp-admin/options-media.php.

Please note: a new image is not created for each post, but instead an <img /> element inserted. Scaled images are generated when you upload the media, not insert it.

In addition, once an image has been inserted using Add Media, you can click the image and then click the edit image icon that appears in the top left of the image. Go to the Advanced tab, and you can set a new size for the image just for that post (a new image will not be generated, it will simply scale the image you selected).

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Thanks for your reply. – user32161 Jun 8 '13 at 9:03
I see your postscript there. I understand what you are thinking now - that a new image file is created for each post. That is not how WordPress works. The files are already there, you are simply linking to them. A new image is not created for each post. – Colin Campbell Jun 8 '13 at 9:48
Is it possible to add ON/OFF button on new media page to control functions.php of adding the script by add_image_size( 'medium', 210, 9999 ); – user32161 Jun 9 '13 at 23:59
Okay. Just to clarify - what you are wanting to do is to select what size images are to be generated for each image that you upload independently? – Colin Campbell Jun 10 '13 at 7:33

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