"Mpeg Streamclip" would definitely be the optimal software for you. Free and professional, you can easily output stills, and if I've well understood what you need (with your other thread), you'll find here all you need to convert any video file without buying editing software.
Exactly. Then double click to open in preview and you can crop, resize and save as jpg. Easiest way to make thumbnails for those antiquated interfaces that can't auto generate them.