It's important to WATERMARK your work for various reasons!
It belongs to you so take the opportunity to Brand yourself/your business.
When you watermark your picture(s) it shows professionalism & that you're not just anyone trying to sell a quick flip.
Watermarking your photos also provides some sense of security. It may be ideal to put some of your logo/watermark on part of the piece you've refinished. You don't want someone else using your work in their portfolio!
Watermark Apps ;
* I'm sure there are several more, however these are the ones that have been discussed on FPBT.