In addition to Peter's reply, there is a way to use the standard XML
SOAP envelope and still get good performance with binaries. Have you
looked into WSE 3.0 and MTOM? Here's something to consider:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/wse3.0/html/434
4d43e-ceb4-43a9-8f8c-6a3f89f786bd.asp
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/wse3.0/html/b4b
19453-e4e4-4056-906d-72504ed8c0df.asp
http://msdn2.microsoft.com/en-us/library/ms733742.aspx

<quote>
To avoid this encoding overhead, the MTOM standard allows for
externalizing large data elements [...] and to carry them with the
message as binary data [...] MTOM messages are packaged as
Multipart/Related MIME sequences with the root part being the actual
SOAP message.
</quote>

There are several other choices, such as SwA (SOAP with attachments, not
supported by Microsoft tools) and DIME, but they are both deprecated,
after being replaced by the superior and much preferred MTOM technology,
now implemented in WSE 3.0. Note, however, that not every client
supports MTOM, and if you can tolerate the 33% size gain, standard
Base64 is still more portable.

I have the same problem as you: I need to return JPEG images from a Web
service, and I'm currently considering MTOM.

Tom