This is the image on which we want to add a glow. Those tubes don't look right this way.

First, of course, you should have your image rendered. But not just the image: you also need a mask of the parts that need to glow. In POV-Ray (2 or higher) that's as simple as setting every object to ambient 0, except the glowing objects (or textures). Make them:
pigment {rgb 1}
finish {ambient 1}
And remove all the light_sources.

The mask of the image above looks like this:

Now open both images in PSP5. (I'm not sure the following technique will work in PSP6!) Make an alpha-channel of the mask-image. This can be done by first making a mask of the image (see picture below, menu mask=>new=>from image, accept all default values), and saving the mask to an alpha-channel (menu mask=>save to Alpha Channel, accept all default values).

Now add a lighten-layer to the actual image (so not to, in this case, mask.jpg, but image.jpg).

Load the Alpha Channel you just created as a selection (menu=>selection=>load from Alpha Channel)

Feather the selection (menu=>selection=>modify=>feather, choose the number of pixels depending of the image-resolution).

Now it should be saved as an Alpha Channel again (so it can be loaded as a mask) (menu=>selection=>Save to Alpha Channel, accept the default values and names).

Now paint the lighten-layer (which should be the active layer) into the color you want your image to glow and load the mask from the alpha channel you just created (menu=>mask=>Load From Alpha Channel, make sure you choose the feathered alpha channel!).

The tubes glow! Doesn't it look much better this way? Of course it's a slow technique and is therefor not suitable for animations, but I'm sure it's not too difficult to write a program that does just that: adding glows to a series of images. In fact, I was just trying to make one. My ideas can be read here (Only for Dutch-speaking guys :-).

Comments, ideas and other stuff can be mailed to me.