Looks like you are loading too much images for the memory you have on your device.

You can debug your sketch with monitor or ddms from adt tools to know the heap size needed by your application.
If you go above the value of your heap size growth limit, then you have that message and the application quits.
you can search for build.prop property to increase thoses values.

thx, very much! But where can I increase the build.prop property? Programmatically or on my device? 

the build.prop is a simple file to configure your android device, with a lot of interesting features.
you can access it with a rooted phone,
not sure that modifying it with processing is the best idea. if you do some mistakes in the file, it can crash your phone.
you can download some apps to modify it with a graphical interface like build.prop editor on the google play.
be careful, and be sure of the real value of your phone's ram.
and use garbage collection to avoid exceed of memory.

Thx! However I'd like to send the program to a friend and therefore change it programmatically. Do you know the code I'd need?

no
I guess it's like changing a simple txt file…but with kind of super user rights. i advise you to use garbage. do you know the heap size needed by your application ?

no, sorry, I have no clue, where can I look it up?

debug tools from sdk are made for this

" You can also reduce the amount of memory that ProGuard needs by removing unnecessary library jars from your configuration, or by filtering out unused library packages and classes."

import processing.core.*; 

import processing.data.*; 

import processing.event.*; 

import processing.opengl.*; 

import java.util.HashMap; 

import java.util.ArrayList; 

import java.io.File; 

import java.io.BufferedReader; 

import java.io.PrintWriter; 

import java.io.InputStream; 

import java.io.OutputStream; 

This is fine-grained compilation. You should only attempt this and other running Java arguments
after you have a final app product ready for distribution.

There are so much more optimizations to hunt down inside your own code source to save memory!

As a little example, many times a short or char data-type is enough for field variables,
rather than using int only.

You'd net 2 bytes off for each time you swap an int for a short or a char!
And if you use a byte instead of an int, you'd spare 3 bytes!!!  

Pay attention if you have some field variables which don't need to be instantiated.
If so, make 'em static! If possible, even final!!! 

Be warned this type of refactoring is just in case you really need to save some RAM!

More importantly, look for useless instances of classes. And remove any redundancy from your algorithms!

P.S.: I've heard about there is a Java's configuration which makes String using byte[] instead of char[]!
However, it's not a good idea since Java loses its Unicode feature!