For creating a watermark:
I’d strongly recommend Inkscape. It has bit of a steep learning curve, but it’s the best vector graphic editor out there. Best of all, it’s free; doesn’t cost you a cent. You can export as PNG, SVG, and a host of other formats.
For applying a watermark:
What photo editing software are you using right now? Because that will affect how you apply your watermark. You could use Inkscape or Gimp for that, but it would be cumbersome (one picture at a time).