All you have to do is click on "Report This Wallpaper" underneath the picture and fill out your email address. When you get the email, copy the link they give you and fill out the form there. It usually only takes a few minutes for the photo to be deleted from the site. I'm really annoyed that I had to fill out that form 14 times, but what she's doing is wrong and illegal.
Here are her uploads:
Uploaded Wallpapers by spanish_girl - My Desktop Nexus
You're only allowed to fill out a copyright claim if you own the rights to the photo, but you can tell the owner of the photo about it. That's how I found out, and I'm really glad they told me.