Most likely a corrupted plist file. Move the com.apple.finder.plist file out of /username/Library/Preferences/ to the Desktop, OPTION-click & hold the Finder's Dock icon and select Relaunch. If that fixes the issue, delete the moved file. If not, create a new admin user account, log into it, and see if the problem persists. BTW, what are you using to view the photos? That app's plist file might be causing the conflict.