>> processing's image rendering

So I assume you're not doing direct opengl calls?  (still having processing doing all the low-level drawing?)

If so, you'll need to modify PGraphicsOpenGL.  Unfortunately this setting is done by an inner cache class so it isn't easy to just extend PGraphicsOpenGL and reimplement something small - you'd have to redo the entire inner class, so might as well hack the source directly.  Here's the approach:

1. copy PGraphicsOpenGL.java into your sketch folder
2. rename it MyGraphicsOpenGL.java
(reload sketch if necessary to see the new .java tab)
3. remove the package statement, and change all occurences of PGraphicsOpenGL to MyGraphicsOpenGL
4. size() like this:  size(640,480,"MyGraphicsOpenGL");

now that you have your own gl renderer you can hack at it:

5. around lines 1045 you'll find the texture filtering parameters - you'd want to change those to GL.GL_NEAREST.  
(you could also find and disable the mip-map generation, but ok to leave that as is, they just won't be used)