In your assets library, select the video file with which you want to work.
On the ribbon, click the Manage tab.
In the Actions group, click the Edit Properties button.
In the Edit Properties dialog box, click the Change Thumbnail link.
From the three options that appear, click the Use A Picture From My Computer option.
Click the Browse button that appears.
In the Choose File To Upload dialog box, select the image that you want to use.
Click the Open button.
Click the Upload Button.
On the Video Properties page, click Save.
