Image in response to an image

hi all,

since i am novice to webdesign i just want to know that how 99designs or alike sites work. I mean how can i upload an image in response to a image or query and show all these multiple images together ? i mean which areas do i need to look into php/mysql for getting it done ? any hints if its not proprietary information ?

Kindly guide me

Edit:

Do i need to use some sort of CMS for a site like 99designs ? i just dont understand the concept, as to how multiple images are posted in response to a post and then kept track of it