Looking down my blog at previous blog posts I've noticed that some of the images are no longer showing. I've done some investigation and this seems to be a problem caused by iPhoto. I use iPhoto on my Mac to store and manage all my photo's, and then publish them to Flickr using the built-in features of iPhoto. If there's a photo that I've published on Flickr which I want to use in my blog, I simply change the permissions of the image on Flickr so that anyone can view it, and then embed the image into the blog post so that my blog pulls the image from Flickr.
What seems to be happening is that every time I publish new photo's from iPhoto to flickr, some of my previous photo's are also getting re-synchronised. It turns out that every time you make a change to a photo on flickr the URL of the photo also gets changed. This means that the link in my blog post to the embedded photo no longer works.
I'm sure there's a way to stop this happening in iPhoto but for now I've decided that in future I'll copy images directly to my blog rather than embedding them from flickr.
When I get round to it I'll also go back and correct all those missing images in previous posts.