ok, i've found IBM reference implementation and got some questions:
media is added using:

 MediaEntityLocal imgSamp = mediaEntityHome.create();
 imgSamp.setName("image"+System.currentTimeMillis()); imgSamp.setContent(getServletContext().getResourceAsStream("/samples/image1.jpg"));
 imgSamp.setDescription("kocio");
 imgSamp.setMimeType("image/jpeg");

in database record is filled in, image content exists but when:

 MediaEntityLocal media = mediaEntityHome.findByPrimaryKey(imgId);
 resp.setContentType(media.getMimeType());
 resp.setContentLength((int) media.getSize());

 byte[] buffer = new byte[0x10000];
 long position = 0;
 int bytesRead = 0;
 while ((bytesRead = media.readContent(position, buffer)) != -1)
 {
 log("bytes read: " + bytesRead);
 resp.getOutputStream().write(buffer, 0, bytesRead);
 position += bytesRead;
 }

copied from emb spec, serwer throws this:
javax.ejb.EJBException: RuntimeException; CausedByException is:
null
org.jboss.ejb.plugins.LogInterceptor.handleException(LogInterceptor.java:357)
org.jboss.ejb.plugins.LogInterceptor.invoke(LogInterceptor.java:195)
org.jboss.ejb.plugins.ProxyFactoryFinderInterceptor.invoke(ProxyFactoryFinderInterceptor.java:122)
org.jboss.ejb.EntityContainer.internalInvoke(EntityContainer.java:514)
org.jboss.ejb.Container.invoke(Container.java:854)
org.jboss.ejb.plugins.local.BaseLocalProxyFactory.invoke(BaseLocalProxyFactory.java:413)
org.jboss.ejb.plugins.local.EntityProxy.invoke(EntityProxy.java:44)
$Proxy102.readContent(Unknown Source)
net.homeunix.andrzejr.web.MediaLoadServlet.doGet(MediaLoadServlet.java:85)
javax.servlet.http.HttpServlet.service(HttpServlet.java:697)
javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:75)

no content? my database is postgresql 7.4.3 (i've changed DefaultDS to other)

root cause:
java.lang.ArrayIndexOutOfBoundsException
at java.lang.System.arraycopy(Native Method)
at org.jboss.media.entity.MediaEntity.readContent(MediaEntity.java:542)
at org.jboss.media.entity.MediaEntity.readContent(MediaEntity.java:526)
at sun.reflect.GeneratedMethodAccessor111.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)

any ideas?