I currently have the problem, that a thumbnail was generated, but the html sets width and height to zero:
It didn't happen before and I've only installed a database backup plugin meanwhile. I didn't change the theme.
Not only that, but there is some crazy CSS on it too [screenshot].
Something really does not want viewers to see the image.
Is it a problem with just your theme? Have you switched to Twenty Twelve with all plugins deactivated?
Can I switch themes for only one page and only for me or do I have to download my blog and test it locally?
(By the way: Wow, you're fast!)
Can I switch themes for only one page
Not really, switching themes will affect all pages of your WordPress site.
only for me or do I have to download my blog and test it locally?
You should work locally, yes, otherwise everyone can see your website switch themes :O
I didn't test it with my theme, but it is very unlikely that it is the problem.
I've just uploaded another image. That worked fine. Then I've uploaded the problematic image with another name - didn't work.
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