Additional info:
This affected image file will be called in an include statement by one of your PHP files (again odds-on it will be in the plugins folder). The call will look like:
<?php include ('img/theroguefile.png'); ?>
and will probably have been added on to the end of an existing PHP file. Again, if you search your installation PHP files for the filename string of the infected image file you found, you'll locate this file more quickly.
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With no other malware plugin appearing to detect this, my next action if I hadn't have resolved this would have been to delete my site and do a complete reinstall.
C