1. A system for displaying listings to a user, the listings being provided by an advertiser, in a subwindow of a browser window, the system comprising: an inventory database containing listings, each listing being for one property, each listing including: a plurality of images of the property; descriptive information about the property; and customized rendering instructions that specify the sequence in which to display the images in the listing and how to display the descriptive information in relation to the images, the customized rendering instructions not being contained in an animated GIF file; an inventory server, remote from the user, connected to a communications network and configured to: receive data requests; retrieve one or more than one listing from the inventory database according to each data request; and format the one or more than one listing into an inventory data string; and an ad server, remote from the inventory server, in electronic communication with the inventory server via the communications network, the ad server configured to: receive information requests for inventory data from the user; send data requests to the inventory server based on the user information requests; receive inventory data strings from the inventory server in response to the data requests; and for each received inventory data string, display the images in the listing in a subwindow of the browser window in the sequence specified in the customized rendering instructions encoded in the inventory data string received from the inventory server along with the descriptive information as specified in the customized rendering instructions.