Diff between Image Server and Content Engine.

Ist Deifference
IS (Image Service) is just Document and Image Repository server, Where as P8-CE (Content Engine) is just one module out of 8 modules. which is more advance to IS. CE include all major functionality of IS. But P8 has 7 more modules like Process
Automation or PE(Process Engine), AE( Application Engine) etc..
Second Diff
Actually Image Service is the base of P8 and the rappers are different 8 modules. More over when you install PE Process IS gets installed. So if you are planning to upgrade from IS to P8 you need basic 3 components CE, PE, AE, and for web access to Content module and process modules you need either of the following JBoss, WebLogic, Web Sphere or Tomcat Application Server. Of course Database in the vital part of any FileNet. Yes, you can access content stored in IS from Java, you can also use ISRA. For that while installing AE you need to execute the AE setup for ISRA instead of general AE. There are 2 setup available for AE one is simple and other is ISRA support.