So you want to make a mod for Dominions? There is absolutely nothing standing between you and making a mod right now. You can do this with only the text editor and image manipulation tool already on your computer and do a perfectly good job and don't let anyone tell you otherwise. Pull up the Inspector and the Mod Manual and lets get started!
You can find a tutorial on sprite editing for Dominions 3 here.
I need to figure out a way to include the example mods shipped with the modding starting pack.