All the sources mention this as if it worked.
Medium size is set to 635x194, but I still get this out:
<img width="100" height="100" title="fjaril" alt="fjaril" class="attachment-thumbnail" src="http://blogg.nassjo.se/klassblogg/files/2012/12/fjaril-100x100.jpg">
As you see, it hard-links to the shrunken thumbnail size. Docs say I should get a class matching the size argument in the img HTML also.
Where should I look, and what to fix?
I'm just trying to get the first post on the front page to display its featured image medium-sized. I'm modifying content.php in the theme. I can search-and-replace, but that would break if the customer changes thumbnail sizes.
The website is here: http://blogg.nassjo.se/
Just made a child theme, and there, it works as it should.
Update 2 I made a child theme for the main site also, but that did (unexpectedly) not make it work. I have to conclude that the_post_thumbnail('medium') only works on the site where it was posted.
I've put the img html in a var and search/replaced to get the medium size and url, and that hack works.
I'll leave the question up for a bit, in case someone can help me find the real reason so I can remove the hack.