Go to the gallery with the video you want to set a custom cover image on.
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Hover over the video file and click on the round magnifying glass icon. An Image Details box will pop up.

In the Image Details box, click on the Upload Custom Thumbnail button.
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A file dialog box will pop up. Navigate to and select the image file that you want to use on your device.
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Click on the Open button to upload your thumbnail image.
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After your image is uploaded, your custom thumbnail will be visible on your video.