You can put all the related images in a specific directory. Let’s say the directory is grid_images and all the images have jpg extension. Then when you want to show the image, you can get the current day of the month and pull the image accordingly.
$image = PATH_TO_GRID_IMAGES_DIRECTORY . date( 'j' ) . '.jpg';
Once you have the image variable you can use this as per your need.
Note: The images should be named as 1.jpg, 2.jpg, …. , 31.jpg