You can judt draw the image with it's "origin" off screen, and move that point.. you don't need to copy the image at all, just use image(bigImage,xpos,0);

At elast that's what I'm doing in a little sketch I'm writing at the moment, and it runs fine at 50+fps at 600x400...

Possibly for really large images, your original way is faster.. the image I was using was only about 3 times wider than the screen.

Here's the code I was using, which looks pretty similar:
Code:

PImage source;
int x;

void setup()
{
  size(600,400);
  source=loadImage("land1.tga");
  x=0;
}

void draw()
{
  background(60,156,255);
  image(source,-x,height-source.height);
  x++;
  x%=source.width-width;
} 

I do the background() because the image has an alpha channel, and is partially transaprent.

things to try to make this faster:

1) not using the background and an image that has alpha.. pre-composite the background color plus the alpha in the image since it's not going to change. when drawing an image that has alpha, each pixel has to be mixed between the background and the image itself. because filling the screen with the image over the background means that things really won't ever change, better to do this once, and then you can nix the call to background (that'll improve speed) and just do the image call. that way, you can just use an opaque image, much faster in general.

other things that can improve speed (just off the top of my head, not necessarily all applicable in your case)

2) you might try using P3D, it may be faster in some cases. you're not doing it here, but unless you use set() with P3D, avoid using translate/rotate etc before drawing the image because it'll have to use slower code because it will think you're trying to stretch the image.

3) you might also try to use set() to draw the image. in some cases, it'll be faster.

4) also make sure you're not using tint() (though it looks like you're ok). pre-tinting images once will be much faster than doingn it on each frame.

good luck!

But tried P3D again and it's now much faster!! Must be partly to do with John's image() suggestion.

Looking good!

hm, it should only be updating the screen when it's good and ready (vsync ala c64 shouldn't be necessary) although if it's taking too long to update the image it may be trying to blit right in the middle of drawing..

also, did you try the suggestion to mix in the background color? you may get a 2x speedup or better by getting rid of the background() call, plus all the time it takes to composite things with alpha. that may also fix the flicker if you use an RGB image instead and pre-composite the alpha channel.