Thumbnails not being generated at correct size

Its probably because you didn’t set a specific height as it appears to be set as 0. Try adjusting the height and try again!

But I don’t want to set the height, I just want to make sure generated thumbnails don’t exceed the width of 540px. So for example in this instance the image (if proportions are retained) would have a width of 540px and a height of 405px (as it shows in my screenshot).

Setting the height value to “0” means a particular height won’t be forced, correct?

No. Set a height value.

OK, so if I set a height value (say 540px) what does that mean exactly:

1. The image just won’t exceed that height and the resulting thumbnail will be the correct 540x405px

or

2. The image gets distorted and resized to 540x540px

1 and 2 if the image is set to 540 by 540px. Try 540 by 420px

That should avoid distortion. If you want to aviod distortion entirely you may want to Crop the image to whatever size you wish using something like Photoshop or Gimp for instance.

The image just won’t exceed that height

Yes. The image should still be resized proportionally (assuming this is not the thumbnail AND “Crop thumbnail to exact dimensions” isn’t checked). So, the largest length will be set to 540px and the other dimension changed proportionally. If you want to try and set a width only, try setting the height to a large number (eg 1024).

Thank you esmi, that’s answered my question.

1 and 2 if the image is set to 540 by 540px. Try 540 by 420px

I’d rather not, since I want to allow images to be uploaded at any dimensions and just the width be forced to 540px. As esmi has said though as long as it resizes proportionally I can set the height to a large number (e.g. 1024).

I understand from other answers that these different thumbnail sizes are in fact only generated on image upload (rather than image embed) can anyone confirm this?

They are generated on image upload, yes.

OK, so if I changed these Media Settings after the images were first uploaded, my thumbnails won’t be generated at the new correct dimensions?

Based on that should I use something like Regenerate Thumbnails to recreate all of these thumbnails at the correct dimensions?

my thumbnails won’t be generated at the new correct dimensions?

Correct. That’s what the Regenerate Thumbnails plugin fixes.

Many thanks! Resolved.