The entire postcard does not need to be light, but you should have some area large enough for a postmark. If it is a glossy, shiny postcard, you might want to spray the area with Krylon matte finish, which is available in most craft stores. This will allow the ink from the postmark to stick to the surface of the postcard better. Some collectors put the stamp on first, then spray. While others, spray the surface first, then put the stamp on the picture side of the postcard.
Some collectors buff the area, to make it slightly rough, so the ink sticks better.
Older postcards are much better, for they do not have glossy surfaces. If the postcard is a picture of a person, you don't want the postmark to cover the face of the subject, so place the stamp in such a way that the postmark won't cover your subject. The placement of the stamp on the postcard is something of an art in itself. Once you have the stamp on the postcard, you send it to the Post Office where you want them to postmark it. If it is a first day of issue, then follow the instructions that you would usually follow for FDIs.