Can macros use custom icons?
Yes. You can place custom icons into 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 or 24-bit plus alpha uncompressed TGA files. Their dimensions must be powers of two up to 512 (e.g. 32×32, 512×128). Note: any images that aren’t square will look squished on your action bar.