Analyzing image data and also lifetime data is often an arduous task. Always make sure that you have taken enough samples to be able to obtain data for a large number of cells. In LI-FLIM you can draw regions of interest (ROIs) in the image to obtain averages of the lifetime in that particular ROI. The STD given is the standard deviation of the lifetime values of all of the pixels in that ROI. Therefore one should always take multiple samples of different regions in the sample. Please see the LI-FLIM manual for more information (Help > Documentation).