If your recipients are both personal and professional contacts, a family photo is a great option. You can add additional photos of pets and office staff if they feel like family to you. If your recipients are purely professional, consider a photo of your staff or yourself (if you are the sole service provider), your marquee, logo or building.  It can be tempting to go crazy with photos on your customized card, but remember, often simple is better.
For photo thank you cards, a picture of your business or staff, or even just your logo pairs nicely with an appreciation or thank you message making these cards highly usable and multi-purpose.