how do select different sizes of pictures?

[Evanrich]

I remember in the default install, and on other peoples galleries, there was a drop down where visitors could select different sizes of the photo. Long ago i turned this off, but now I would like to re enable this feature, but do not know what is required... does anyone remember how to do this? I did a fashion show, and I want to display compressed jpgs for the normal visitors, but for people with faster connections, i want to upload full size jpgs that are about 600KB a piece. They're already in the upload folder, just don't know how to get it so i can select which size when viewing a photo.

Thanks for any help in advance.