All of the covers I have produced for Keith's albums are constructed in a similar manner. I take a set of images and layer them on top of each other. I apply graduated transparency effects so that across the image one picture fades out and turns into another. I combine this with cut-outs, such as the letter E used in the cover for Echoes.
This sort of technique requires pictures with a good range of contrast. Real photos work well.
Usually, it is also a good idea to have one image that holds the entire space together, and image that appears in the gaps or dark areas left by the other pictures. Swirly patterns, such as a star nebula, or those generated by KPT Texture Explorer work well. You'll see others use graph paper-like grids, or sets of concentric circles or radial lines. Sorry about the copyright symbol on the following, but I really need to enforce my copyright on this one. I hope you can still see how the grid of white lines (look at the top left-had corner) and the swirly pattern hold the other images (staircase, tunnel, hook, lightning) together.
I think these elements serve much the same role as a rhythm track, they provide a pace and structure to the work. You can choose to view it as a whole and let it wash over you, or you can peer at the elements, taking in the details. This is why photos work best: there need to be details as well as pattern.
Perhaps it won't surprise you to learn that I have 8 drawing tools readily to hand on my desktop, and another 8 that I use occasionally for specific tasks. Each has its strengths and weaknesses. The image above, and all of the images for the album covers were created in just two of them: Paint Shop Pro, and Xara X. Paint Shop Pro is handy because it can host filters designed for use with PhotoShop (which is way too expensive and awkward for me). In the image above, the swirly pattern was created in KPT Texture Explorer hosted in Paint Shop Pro, and the rest was done in Xara X. The images for Linden Wood were created entirely in Paint Shop Pro; I only used Xara X to add the lettering and cutting guides.
What makes two these packages useful is that you can build up an image in 'layers'. This is a bit like printing all your photos on tracing paper. Then you can move them about and apply effects to the different layers separately.
Here's one set of tests for the 'god' idea for Linden Wood (I warned you it wasn't pretty).
The images are drawings of gods, with some goldy texture applied in places. The goldy texture is created with Flaming Pear's SuperBladePro. Each image is on a layer: I've included the layer palette for you to see. Each layer can be turned on or off (the spectacles). Each layer can also have 'filters' applied. As you can see, I am experimenting here with Dodge and Difference. Layer 2 contains the image in the middle that shows grey against the white background. You can see how the Difference filter uses the grey and white areas of layer 2 to modify the pinky gold images beneath.
The other set of tests were an idea I had for the 'swirly pattern' picture. I am interested in decorative paint-effects, so I thought it would be interesting to see what these would do when scanned in and combined with photos. I made a few test pieces using different colours and effects and scanned them in. Here's a couple of the tests:
I've used a heavily textured paper for these tests. The colour is ordinary water colour paint, and the metallic paint is the stuff they sell for home-decorating stamp effects. The technique is called wet-in-wet. You leave the water colour very wet, then just touch the surface with a brush dipped in the metallic paint. The metal spreads out, woosh, across the water, leaving soft wispy edges when it finally dries.
I also tried some other things, such as printing images out before applying paint and scanning them back in. Interesting, and I may well use them for something else in the future, but they did not seem to be going in the right direction for the current project.