Create unmarshaller with the following xsd:
<?xml version="1.0" encoding="UTF-8"?><xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:jaxb="http://java.sun.com/xml/ns/jaxb" xmlns:xop="http://www.w3.org/2004/08/xop/include" xmlns:xmime="http://www.w3.org/2005/05/xmlmime">
        <xs:import namespace="http://www.w3.org/2005/05/xmlmime" schemaLocation="xmime.xsd"/>
        <!-- as a value of JAXBElement -->
        <xs:element name="png" type="xs:base64Binary" xmime:expectedContentTypes="image/png"/>
</xs:schema>

and unmarshal the following xml:
<?xml version="1.0" encoding="UTF-8"?>
      <png xmlns:jaxb="http://java.sun.com/xml/ns/jaxb" xmlns:xop="http://www.w3.org/2004/08/xop/include" xmlns:xmime="http://www.w3.org/2005/05/xmlmime">iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAAkElEQVR42qWT0Q3AIAhEGcRBXMRBHMRFHMRBXITmkmosRTHhEj6qxwtQJCJiZxDXWnkVvldTSomleu8cQrABRvIZABPMh+Q94DJ5D5BnENq5GuJOMUYfAC24AFDO+R6gnStV6IDxK7U7UcXf1FqbBgzOqMJeZQClSin3AK0KaL4F9DSWB6FM+nM/4l0s33N+AJVplWD8cBu5AAAAAElFTkSuQmCC</png>

and then marshal back, but got png content:
iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAAmElEQVR4XqWT0Q3AIAhEGcRBXMRBHMRFHMRBXITmktIoYtV4CR+V4wlqiYj4MohzztwK360phNDloVorO+fWgEXxPwAmmFup4jlgs3gO0GsQxlHFNmAm770uPgNghCsAFGPcB1jrRhc2QK7SyqkuRlMp5TPg4LRUFyNAdpcAUCultA+wuoC+fwEzyeNBGCfd5SXeh9UbT+MBlWmVYI2LyuwAAAAASUVORK5CYII=