go to the game page, then scroll down to the pictures. There's a link saying "Add Picture". Choosing this will prompt you for the filename on your computer & a description. The image file will get copied up to BGG and will be shown after a slight delay (maybe minutes, maybe days) while it gets approved by admin.
Go to the gamepage for the game you want to submit the photos. In the photo section of that page, there should be a link , or . The image will then go into holding area where an admin will approve or delete the image.