I am a bit rusty with pub.client:http, but HTTP can be used to transport binary data just as is (i.e., it is 8-bit clean). I think you need to convert to bytes and set it to the data/bytes input. Of course, the format of the POST would depend on the receiving service. You can also put binary data in the CDATA section of an XML document, but I am not sure if webMethods can parse that.
#webMethods#Flow-and-Java-services#Integration-Server-and-ESB