To load a non time-gated image (for instance, an image acquired with normal illumination and detected without gating), use the File>>Load>>Other Files>> Open While Light Image menu item.
This launches a different dialog than the one for time-gated data or time-series, which allows previewing the file to be opened:
The supported image file formats are TIFF (8 or 16 bits), BMP, JPEG and PNG.