If you would like to change the preview image for a video, first, select the video from the gallery and open it in lightbox.
Click on the wrench icon > Replace Preview option.
You will see that the preview image has certain size requirements, you will need to meet the EXACT pixel size to be able to replace the preview image. Open a photo editing software on your computer and make sure to use the very same dimensions for the preview image, save it as a .JPG.
Once you saved the preview image on your computer, select 'CHOOSE FILE' and pick the preview image from your computer's directory.
Voila! There's all to it - now your video has your preferred preview image added as the image thumbnail :)
If you would like to display a very simple play button, I created two files you can download below - just click on the image, than right click save it to your computer :)