1. An image display method for use in a network system in which image data items are uploaded from a plurality of user terminals to a server and images are opened to public among the users, the method comprising: storing a plurality of image data items uploaded from the terminal as they are or after subjecting the image data items to image processing as works for publication; classifying each of the works into one of a plurality of categories; and performing category control to execute one of category unification, category division, and category deletion in accordance with a number of works included in each category, wherein the categories are stratified into a multilayer structure, and performing the category control includes (i) when a category is to be unified with some other category, absorbing the category into a one layer upper category, to unify the categories together, (ii) when a division of a category is to be executed, dividing a category out from another category, forming a one layer lower category, and (iii) category deletion which occurs by deleting a category.