GIF format files are a proprietary to Compuserve, Inc. Many development tools and services (such as the ones used by SpinSite.com) were forced to remove functionality to manipulate images with this format.
You may still resize your .GIF files using your client image software and re-upload it to your SpinSite site. Or, you may leave the file as it is and have it displayed smaller. This is an option when you add or modify an Image feature on your site.