Is This a Web Service?

The recent conversation has included the interesting idea of looking at
Web services as SOAP services.  Are we really saying that SOAP is
integral and required for ALL Web services?  For example (and this is a
real example), suppose there is a Web site on the internet that is
oriented toward returning results to people on browsers, but if you set
the parameters of the GET in a particular manner it runs an application
that generates an image (the nature of which depends on other
parameters) and returns that image (and only the image) as an HTTP type
image/png.  I now have an application that at some point wants to make
such an image with the contents based, shall we say, on calculated
values (in fact, this determines text that is inside the image, if you
must know) -- and I do a GET with the appropriate parameters, wait for
the HTTP to come back, write the binary stream of the image somewhere
and go about the business of the application which does something with
the image.

Now, I personally think that's a Web service, mostly because of the
application to application flavor.  I would not call it a "W3C Web
service", since it doesn't use WSDL and SOAP -- but it seems pretty Web
service-ey to me.  I would personally call it an "ad hoc" Web service --
and I would make up another name for ebXML transactions that use SOAP
but not WSDL, since it seems to me that those, too, are Web services
that handle the description differently. 

But what do you folks think?  Does it absolutely have to use SOAP to be
a Web service?  If so, that's an interesting and really useful thing to
know.  My personal opinion, for what it is worth, is that simpler, ad
hoc things like the example above are, indeed, Web services, but you
quickly start needing SOAP if you want to do anything other than the
most basic operations, and so in practice most of the "interesting" Web
services use SOAP.  I am certainly willing to agree that if a Web
service uses ANY enveloping mechanism that it should be SOAP, since
there don't seem to be any other real popular candidates.  Is that a
reasonable point of view?