A Shorts cover workflow can depend on the device, account, and publishing interface. Do not assume a downloaded image can be selected in every YouTube app screen. Open your actual publishing workflow first and confirm which options it offers. If it asks you to choose a video frame, image preparation alone does not put that frame inside the video.
YouTube's thumbnail help describes current custom thumbnail options and eligibility. Refer to that guidance and the controls visible in your account rather than treating a general image preset as permission to upload a separate cover. The preset here produces our 1080×1920 vertical export; it does not unlock an account feature.
Choose artwork that honestly represents the Short. A dramatic crop that hides the actual subject can become confusing when someone opens the video. If you need a designed frame inside the video, use a video editor to include it and check that the publishing interface can select the intended frame.