Creating shadows under units
by Richard Kettering (Jetrel)
Wesnoth has been through several iterations of how shadows are drawn, our most recent being a slightly blurred version.
Our current standard for drawing shadows is fairly simple. All of the shadow pixels are given an opacity of 60% (or in other words, a transparency of 40%). The preferred shadow color is RGB: 22,0,52 (on a scale from 0->255; in hex it would be 160034). This has minor cosmetic advantages over pitch black (rgb: 0,0,0), because most shadowed surfaces in the real world have a large amount of blue-ish ambient light reflected into them, rather than being just a darker version of their source color. It also makes shadows easier to work with in mixed-down PNG files, because you can select just the shadow by clicking on it with the magic wand tool (since it's a different color than black, which invariably ends up getting used on the bodies of most units).
To create a shadow, take a unit you wish to apply shadows to.
Create a new layer (we're assuming here that you're using Gimp/Photoshop or some layer-supporting graphics program), and draw in it, with the pencil tool set to RGB: 22,0,52 the desired shape of the shadow you would like. Note for most units, that our perspective will make the shadow of most creatures somewhat wider than it is tall.
Finally, move this layer underneath all your other layers, and set its opacity to 60%.