Right way to define a page thumb when posting/liking site on facebook?

Im building a site and one of the things I’ve added was the facebook like button, a not-too-much-invasive feature asked by the client. But my trouble is that everytime I ‘like’ the page online, the picture displayed on the dialog box is not the one i wanted… So, I tried putting a image with a full url and a “display:none” attribute so facebook could read it and show it. No again. So I searched some dev blogs and found a metatag was claimed to do the job: <link rel=“image_src” href=“image.png” /> but it didnt. I Tried <meta property=“og:image” content=“image.png”/> but it failed too.
The thing is, it shows a image, but I dont know how facebook gets the image: alphabetically, by tag, or a defined way, so i can prepare my site with an image and facebook recognizes it and display it