The *Chaoscopers main page mentioned wanting a tutorial on this, and I immediately realised how it could be done. In approximately three steps. ^_^; It's really easy and simple.

Ok, so. You've made a nifty light/plasma render in Chaoscope. But man, that black background is boring! How can I put something else there, without losing the transparency?

I'm gonna tell you how. :3 This assumes that you're using Photoshop, and that you know a bit about how to use Photoshop. It will also work in Paint Shop Pro, and any image editing program with layers that allows you to set the mode of those layers.

First of all, find/make your background image. It should, of course, be the same size as your render.

Now open your render. Select the entire image, and copy it to the clipboard. Switch to your background file, and paste it in as a new layer.

Ok, so now you have a completely covered background. This isn't what you want! Assuming Photoshop, on the layer pallette, you should see a drop-down box that is currently set to Normal. Click the arrow, and you should see an option for 'Screen'. Select this. Bam! The black drops out and goes away, and your background shines through. But your render is still there! And lo and behold, it is still perfectly transparent! Depending on your background, it may look a little washed out. No worries, just use the sponge tool, set to saturate, at a 999 pixel or so hard brush, and tap it on the layer with your render a couple of times, and the colours should pop right out. You can also dodge/burn if you want it a little lighter/darker.

You thought that would be hard, didn't you? n.n

You can also screw with the texture/colouring of a render by pasting it over a pattern or a gradient, and setting the layer mode to multiply.