I have just had another stab at this. Following a fresh compile of
Apache 2.2.0 I'm still getting the same odd behaviour but looking in
the access_log I see error code 206 for each image. This error is
defined as follows:

"206 Partial Content

The server has fulfilled the partial GET request for the resource. The
request MUST have included a Range header field indicating the desired
range, and MAY have included an If-Range header field to make the
request conditional.

I'm really none the wiser... Anyone have any ideas?