So, when you click on the thumbnail, the page scrolls to the top and you need to wait until the wallpaper creator load the resources. Then you will be able to customize it.
If you do not see the loading progress bar on the top of the page after a while, do the folowing:
press F12 and copy the contents from the console tab, past it here. If a error happened it was logged in there.
Cum aș putea să îmi fac propriul meu walpaper ???