You can make any custom images, and put them in the /Interface/Icons/ folder, and use them in macros as well. -- so long as they follow certain rules.
If you would like to use custom icons for your macros, you can place them in your World of Warcraft\Interface\Icons folder (creating this folder if it doesn't exist) . The files must follow the same guidelines for UI textures . Namely, they must be either BLP files or 24-bit/24-bit+alpha uncompressed TGA files. Their dimensions must be powers of two up to 512 (e.g. 32x32, 512x128). Note: any images that aren't square will look squished on your action bar.
From the UI forums sticky :D