We are building the grids/tiles with the first post/page/featured image found.
To show the pictures in the grids, we need at least one of these items: (1) gallery , (2) post image , (3) Featured image.
New: if a (6) Vimeo or YouTube URL is found in your post, we will display an image which they provide in the grids/tiles.
To gather your grids, store them in a (4) Category or (5) Tag, means that you can make a grids/tiles collection from a certain category or Tag.
If there is not any image available we use an default one which you can change in the settings of Grid WUD. (read more here)