1. Here is the standard background I use:
2. Taking a screenshot of the unit in Atlas with the "Big Screenshot" option, it looks like this (note, I use a custom scenario map with a bright ground texture for ease of cutting in Photoshop)
3. A. I cut the unit out of the background, B. copy the unit into a 2nd layer (this is important), C. resize it to 256x256, D. then add the background texture at the top of this tutorial. E. From there I adjust the Layer Styles (effects) of each of the unit layers to give the final product. This is what the file looks like for me:
4. These are the layer settings I use. First, to get that nice brownish background glow (THIS IS WHAT IS MISSING FROM MANY OF THE NEWER PORTRAITS IN THE GAME):
5. Here, I create the drop shadow effect:
6. Sometimes I add a subtle Bevel and Emboss to help the unit stand out a bit from the background. THIS IS SUBTLE, and usually used on a unit with dark edges, like dark hair or a dark horse. Sometimes I don't even use it.
As an example, I used this effect to make the Balearic Slinger's head stand out a bit more, otherwise it would have blended into the drop shadow.
5. Here is the final product:
Further Examples:
unit_portrait.psd