Thanks for suggesting I look at the version of processing I was using. I just upgraded to processing 2.0.3 (from 2.0b8) and this has fixed my issue so that there are no problems at all. 

My initial reasoning that this code should work was that :

PImage screenImage;

 was defined as a global variable, and so each time I used:

screenImage = loadImage("screen.png");

the new image would be loaded into the screenImage variable memory space, not take new memory! Anyway, I do concede to the point that doing it 30 fps is wild excessive, thanks tfguy44 for your code to reduce the loading rate, I only need once or twice a second. I wanted to check before implementing something like this though that the underlying problem was solved - so I was not just increasing the time it would run for.

Thanks again,

Jon

Due to the background(255) this just flashes up the images every .5 secs and displays a white screen the remainder of the time.

I moved this up into the setup() and this does the trick nicely.

Thanks for your help.

Jon