#Europeana has a rich repository of images of many, many monuments. They show what monuments looked like once upon a time.
Wiki loves monuments is a photo competition where people are take pictures of monuments what they look like at this day and age.
The TimeMash application adds a dimension to the pictures that can be taken; it helps photographers make pictures from the same angle. The combination is powerful; pictures taken in this way are more interesting.
It is possible to have both images on Commons and even used in the projects. The challenge is to register and link such pictures and to have a tool to switch between such images.