If you want to add a thumbnail to a video on your iPhone, there are a few steps you can take. First, open the Photos app and find the video you want to thumbnailize. Then, drag and drop the video into the Photos app. Once the video is in the Photos app, select Edit > Thumbnail. In the thumbnail window, you’ll see a list of options:

  1. Choose which image to use for your thumbnail: A photo from your Camera Roll or an image from your Videos Library.
  2. Set dimensions: The size of your thumbnail will depend on how wide or tall you want it to be.
  3. Add text: If you want to add text to your thumbnail, choose Text > Text Size and enter a size that’s comfortable for you (in pixels).
  4. Save: When you’re finished creating your thumbnail, save it as a PNG or JPEG file and then close the Photos app.