When adding a graphical watermark, it's a good idea to make it look subtle by lowering its opacity so that it is just barely visible. This will make it look less overwhelming – you want the viewers' attention to focus on the image but not the watermark. Many photographers find that 20-30 percent watermark opacity makes it not too intrusive but still visible. However, it really depends on your approach, and you should see what works best for you.
To make your watermark semi-transparent, open it in any image or photo editor such as Inkscape or Photoshop and change its opacity (transparency) value, for example, to 30%.