Video & Poster Image Files
Before importing the video into EverWeb, control (right) click the video file in the Finder and select "Get Info". Note the width and height of the file.
Launch the video in QuickTime Player, stop it at a representative frame and take a screenshot. Crop this to the same size as the video file or, at least, to the same aspect ratio.
Aspect ratio is equal to the width divided by the height.
If possible, play the video at fullscreen on a computer with a large monitor to see if the quality is good enough. If not, remove the fullscreen button from the video controls by checking the appropriate box in the widget settings.
Reduce the video and poster image file size appropriately for use on the tablet and smart phone versions of the site.