Thanx for your both of your replies!

AgentSmith:  Thanx for the tip on the render clouds filter and I didn't realise Bryce would read .psd files directly either. I'm using Photoshop v9 and Bryce 5.5

Rickymaveety: It'll be a seamles texture, but highly detailed so I'll use a large image size. I'll have a go at adding an alpha channel to use as the bump map. 

If I create the grayscale image (how do I do this in fact??), then can I make the 'bumps' higher by, say, increasing the contrast of the greyscale, so there is a greater difference between the light and dark areas?

Cheers,

Darren