Downblouse is a term used to describe images, enjoyed by voyeurs who like women, showing the view down a female's shirt or blouse. There are many websites that promote this as pornography. Taking photographs of someone surreptitiously is an offense in some countries, but in others there are no laws to prevent this. Some people consider this sort of photography to be a form of sexual abuse and an invasion of privacy if it is done without consent.
An alternative form of this type of voyeurism are upskirt images - similar to downblouse but with the intent of showing a woman's lower half.
The advent of cell phones fitted with cameras (aka camera phones) may be reason for surge in these types of photographs.