On platforms like ebay, product pictures are an important factor for the sales succcess of your offers. So, in the first place, they are meant to stimulate customers' interests. Though, additionally, ebay itself has specific guidelines and standards that each of your uploaded images has to meet.
FTP is short for File Transfer Protocol and stands for a standard network protocol used for the transfer of files. FTP servers are thus used for the exchange of data, for instance images, between individual computers.