Update: It seems there's an outdated plugin for doing what I want to do: 

https://community.elgg.org/plugins/550231

It seems very small and simple to update. Perhaps I can revive it?

https://community.elgg.org/discussion/view/1622288/german-language-thumbnail-image-file-problem

There are no characters (nor even a newline or whitespaces) allowed before <?php. If there is any content before, how should it be handled correctly? It's won't get interpreted as php code (that's only the case after <?php). Instead of handling it as php code it will be interpreted as content to be displayed somewhere and this can result in unpredictable problems. Therefore just don't put anything in php files before <?php.

The Byte Order Marks (BOMs) are a special case of characters before <?php. The BOMs are non-printable characters. So, you won't see them in an editor. But if they do exist at the beginning of the file (due to saving the file with a BOM encoding) you will get the same problems you would have when putting any visible characters before the <?php. Therefore, you always need to save your files without BOMs, e.g. with "UTF-8 without BOMs" encoding.

Great, thank you! This is good information to have; perhaps it could go into the Elgg documentation somewhere?

The cause of this problem was a newline before <?php in an English language file. PHP apparently doesn't like this, and it causes problems elsewhere. Why, exactly, this affected only images I can't say, but at least I know the cause.