Error when calling an image transform from matrix field I'm trying to call an image via an image transform from a matrix field.

So when I do this:

{% if block.type == "standardImage" %}
  {% set image = block.imageFile.first() %}
  {% if image %}
      &lt;img src="{{ image.url('generalImageTransform') }}" width="{{ image.getWidth('generalImageTransform') }}" 
      height="{{ image.getHeight('generalImageTransform') }}" alt="{{ image.title }}"&gt;
  {% endif %}
{% endif %}


I get this:

&lt;img src="http://.../cpresources/transforms/1?x=DrK2WYEvg" width="600" height="601" alt="district"&gt;


So you can see the width, height and title are all being called successfully, but the image is broken on the web page.

And when I skip the transform and just use {{ image.url() }} the image is called successfully with 

&lt;img src="/images/general/district.png" width="600" height="601" alt="district"&gt;


Finally when I add this to the config file

'generateTransformsBeforePageLoad' =&gt; true,


I get a PHP error

Image “district.png” cannot be found.


I suspect then that the transformed image is unable to be created, but I don't know how to progress this idea, or if I am on the right track at all.
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