media-library: how to insert thumbnails with a fixed width, regardless of landscape/portrait orientation

You can set a maximum width for all image sizes and WordPress will crop portrait and landscape oriented images to that same width.

In your media settings set the width field and leave the height field blank.

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When you upload a landscape oriented image the size will never go past the max width of 300px and the height will be proportionate .

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When you upload a portrait orientated image the max width will still be 300px.

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When the post or page is displayed both images will line up and be the same width:

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