I believe you misunderstand how it works. The user has to have downloaded the image either from a server or otherwise, in which case they have probably already downloaded the map anyway. It's not something you can just make public for them to see to draw interest.
okay so how to i put my map picture up after they have connected to the map exactly. I mean i dont want that big boring gray box.
...you can't. People who already downloaded your map won't automatically download the menu thumb image. You can create a .res file for future releases of your map and include it there so the images will be downloaded together with the .bsp file. The .res asures that TF2 knows which files should be downloaded and can be used for files that can't be pakrated (like intro videos and those menu thumbs).