Besides cropping photos, you also have the possibility to specify a 'preview frame' for videos uploaded via the Asset Manager. After uploading the video, click the cog wheel icon under 'Options', and then click on the blue 'Edit Thumbnail' button:
Choose any point or frame in the video and click 'Save thumbnail' when you're satisfied.
Processing the thumbnail may take a few moments, during which you will see a message that the thumbnail is being processed.
Click 'Save' to exit the Asset Manager and go back to the editor.