Something's wrong on your computer, unless the files are corrupted or lack proper suffix (.jpg). Are these from e-mails? Did you try to trash Preview preferences?
I have never heard of Viewer nor have it as part of the OS? Any details about this app?

Alex
3.1 GHz 13" MacBook Pro 2015, 8 GB RAM, OS 10.11.2, Office 2011, TimeWarner Cable
2.8 GHz Xeon Mac Pro 2010, 16 GB RAM, OS 10.11.2, Office 2011, LAN