Statas - There is a V4 Script that will "pull" stills from a selected track, at your predetermined SET intervals. It is arbitary - ie you don't select them, the script does. I've used it and then used Alexie's Still To Markers script. I placed the markers to on-the-beat parts of a the audio. This then gives stills appearing on the beat - neat eh?
Run Add Markers At Interval (you will need to modify the code to set up the desired interval - it defaults to 5 minutes) This will place a markers on the timeline at the specified interval.
Run Thumbnail At Markers which will create a series of images - one from each marker location. (Once again, you will need to modify the code to specify the output subdirectory - defaults to D:\VMedia)