A bit more information about this problem. When opening an image from within Everything, ImageGlass is using igconfig.xml, igstartup.profile and the Themes folder from the C:\Program Files\ImageGlass folder instead of the %AppData%\ImageGlass\ folder.
I am guessing that igconfig.xml, igstartup.profile and the Themes folder located in the C:\Program Files\ImageGlass folder would normally be used for a portable version, but this is NOT a portable version. It's an installed version.
A workaround for the problem is to copy igconfig.xml, igstartup.profile and the Themes folder from %AppData%\ImageGlass\ to the C:\Program Files\ImageGlass folder.