How do I need to identify a dress I found online?

I found a picture of a dress I liked on facebook. I have the picture but I don't know what brand it is or where I can buy it. I really want to wear this dress to my winter formal but I dont know where I can find it. Any advice on like a website or app that can identify the dress through the picture, or any other ways i can find out where to buy it? thanks! oh a btw I cant ask the person because I dont know them