I just got a new theme and am trying to regenerate thumbnails. I'm having trouble though. I tried using the AJAX Thumbnail Rebuilder but for 5 hours it just said reading attachments. Then I tried the normal rebuilder but the vast majority of images came back as undefined.

Any ideas?

  • Have you enabled post-thumbnails in your functions.php file? – Dan Gayle Jun 1 '11 at 17:15
  • yes I have, I have thumbnails showing up on the site, just not correctly (dev.wegotthiscovered.com) – Matt Jun 1 '11 at 21:00
  • even for the ones that it said it re-sized, it did't, not one image worked – Matt Jun 1 '11 at 21:45

i have only had to do this on a couple of occasions, but each time i have used this regenerate thumbnails plugin on each occasion with a 100% success rate, however (as you will expect it did take quite a bit of time as i had over a 1000 images that it had to wade its way through on each occasion.

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  • I did this and out of 6000 images, about 3500 came back as undefined, any idea why? – Matt Jun 1 '11 at 20:59
  • 2
    did you try this: jdhodges.com/2011/05/… – MartinJJ Jun 1 '11 at 21:12
  • No but all my images are in the same folder, so if it can access some with the 755 permissions, shouldn't it be able to access all? – Matt Jun 1 '11 at 21:14

I find the AJAX Thumbnail Rebuild to be better than the regenerate thumbnails plugin, because it doesn't tie up your server as bad, allows you to skip certain sizes (which saves time), and gives more visual feedback.

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  • I tried it with only doing one size and still, for hours and hours it just showed me that message "reading attachments" – Matt Jun 1 '11 at 20:59


From my personnal experience, and about half a million other people, Force Regenerate Thumbnails is the best thumbnails regenerator I have ever used.
Hope this will help you.

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