Magento Category Images not Appearing in Frontend

I am developing a Magento CE 1.9.1 site and am having an issue with Category Images not appearing in the frontend. After a log of Google searching and code hunting I am not totally sure how the Category image cache code works. Can anyone explain how it works?

I am able to create my categories as sub-categories of the root category. I am able to upload an image using the "choose file" button on the /admin/catalog_category/ page. When I look at /media/catalog/category/ I can see that the image has been uploaded.

When I look at the site frontend and I go to the category for which I have just uploaded the image I am seeing the default Magento blank image rather than the image I would expect to see.

When I examine the image I see the img tag has the src = /media/catalog/cache/3a92671e90bc216c9cce162eaebee21b.jpg

When I look in /media/catalog/cache/ I can see that this image is the only image there and when I look at this cached image I can see that it is the default Magento blank image. Note that when I remove this image from the cache directory it is recreated when I refresh the page where the category should be seen.

Note that when I click on the blank image I am taken to the expected content for the category.

What I can't work out is

1) Why has my uploaded image not created an image in the cache directory? I would have thought it should have been created when I visited the page where this category can be seen.

2) How do the image src filenames get created?

3) How do these file names get linked to the particular category?

Can anyone offer suggestions on how I can fault find and hopefully get this category image solved.

My server is Ubuntu 14.04 and Apache and PHP are working fine. A few other Magento sites exist on the site and are working fine. I have opened up file and directory ownerships wide open so I do not believe this is a permissions issue.

Kind Regards

Richard