how do I open a .sig file?

If you just changed the extenstion from .tiff to .sig that will not work.  Need to change it back. 

SIG is an image file format created by Broderbund. SIG files hold images used for signs or posters in a proprietary format.SIG files can be opened and edited by Broderbund graphics software, such as PrintShop and Print Master. SIG files cannot be opened on non-Broderbund software, but can be converted to .JPEG form using an image converter.