She is hot but I don't recognize her. Without a webpage for clues and/or search words, or some info in the photo properties, it is very hard to find a girl unless someone recognizes her.
About the thumbnail, I am assuming you mean using her pic as your avatar, or the picture under your username when you make posts? To do this click on your username (after signing in) at the top of the page by Welcome back, and then click on avatar and then resize a pic of her to appoximately 90 X 90 and then upload it to our server by clicking the choose button then steering it to the photo on your computer.
If you meant having a thumbnail show up next to a THREAD that you make, this is automatically done if the link in your post leads directly to a pic gallery. The first pic will then become the thumbnail.
F~