[jitter] first shader

your problem is most likely in this line:

  <param name="image" type="float" default="0" />

Change type to "int" and you will probably be okay.

FWIW, I would recommend ditching all of that conditional stuff (if...) 
and replacing with mix() and step() type functions.  Support for 
conditional statements is pretty unpredictable in GLSL.

Best,
Andrew B.