I can't say for sure how Arwen did it, but there are several ways of getting colour overlays on an image.
The first is filling a new layer with a colour, green in this instance, and putting putting the layer setting on something such as 'overlay', 'softlight', or 'colour'.
If you fill it with a lightish colour 'mulitply' also works well.
Another way is to go to the hue/saturation control. In my version of photoshop you get there by going enhance>adjust colour>hue saturation. It's not the same in every version, although going ctrl+u is a shortcut which does.
Anyhows, once you've got into it, make sure that the 'colourize' box on the right is ticked, and fiddle around a bit.
A third way of doing it is using levels. Go enhance>brightness/contrast> levels (or ctrl+L). Usually you just leave it on RGB, but if you're wanting to change the colour then you'll need to fiddle with the red, green and blue individually. To get a greenish colour decrease the red (drag the arrow on the left, and possibly the middle one, right a bit) and increase the green (drag the one on the right and maybe the middle one, left.) Just fiddle round until you get what you want.
My version of photoshop doesn't allow you to fiddle with curves, but I think PSP (which is what Arwen uses) might. Of the three things I suggested, the last probably gets closest to the A-U main pic. But she may have also used a texture, or a combination of things, or something totally different. I hope what I've said is a little bit useful, though. And Arwen, if you read this, please do tell how you got the effect. I'm really curious.
