I use an online service to download youtube video thumbnails with correct dimensions, all these dimensions follow youtube's pre-defined algorithm. The dimension of youtube HD image is 1280X720 Pixels. it may be helpful for someone.
You can use online service like (https://www.youtubethumbnaildownloaderonline.com/?id=4Jj58ObtgaA) to get the preview image of your choice
You can use online service like (https://www.youtubethumbnaildownloaderonline.com/?id=YQB8i4XjfX8) to get the preview image of your choice