Use the writeImageContent method for the Picture object. Documentation here.
writeImageContent
public void writeImageContent(java.io.OutputStream out) throws java.io.IOException
Writes Picture's content bytes to specified OutputStream. Is useful when there is need to write picture bytes directly to stream, omitting its representation in memory as distinct byte array. Parameters: out - a stream to write to Throws: java.io.IOException - if some exception is occured while writing to specified out