To set zoom sized images you will need to select the zoom option prior to uploading here:
Website Controls > Manage Pages & Galleries > Click Edit beneath the appropriate gallery > Upload Media Tab > Presentation & Protection section

If you've already uploaded the images you may re-upload the images without having to re-enter your captioing and other settings and set the zoom setting here:
Website Controls > Manage Pages & Galleries > Click Edit beneath the appropriate gallery > Edit/Delete Media Tab > Replace Media/Thumbnail Tab