One you have a product image loaded for the thumbnail of the video you wish to display,and your video is uploaded to Youtube, in the Youtube settings for the video, click the "Share" button that is underneath the video
Then Click "Embed"
This will expand a section that generates the embed code for you. You can use the options to adjust the display width.
Now go to the Product Editor for the product you wish to add the video to and go to the Photos tab.
From here, you'll want to click New in the top right.
Then click the Insert From button.
You'll need to attach a picture to the video before you add the video.
Tip For Video Thumbnail Images
This can come from a source on the internet, or from a file located on the store itself.
You can take any image you wish to add as a video thumbnail image by adding a transparent play button to the image in any image editor of your choice.
Example of transparent play button:
Now go to the newly added Photo and do two things:
1. Check the "Is Video" check box.
2. Paste the Embed Code from Youtube in the Video Content box:
Now there will be a thumbnail for your video and when clicked it will popup the video just like a product photo.
Note: the video option will only work for images that are not marked as "Is Primary".
Check out these other Articles on Videos: