Error when calling an image transform from matrix field
I seem to have worked it out - by specifying 

'imageDriver' =&gt; 'gd'


in the config. I first tried to specify 'imageDriver' => 'imagick', but I was getting path error messages so I'm guessing it is not installed on the PHP installation I am using.

More info at
Errors with Craft 2.3, Imagine and Imagick