The recommended 유튜브 YouTube Shorts thumbnail (vertical) spec is 1080×1920 px (9:16). Shorts are 9:16 (1080×1920). Shorts thumbnails are picked from a frame, so make your first frame at this size.
"Fill" center-crops when the ratio differs. "Fit" pads with the chosen color. "Fit width" keeps the height ratio, for detail pages.
Drag the preview of a "Fill" result to move the crop.
JPG, PNG or WebP, quality 50–100. Each result card shows size and weight.
Select several files to convert them to the same spec and download with "Save all".
1080×1920 px, ratio 9:16. Shorts are 9:16 (1080×1920). Shorts thumbnails are picked from a frame, so make your first frame at this size.
Choose "Fit (padding)" to avoid cropping; with "Fill" you can drag the preview to move the crop.
Upscaling beyond the original blurs. Use the largest original you have; when downsizing, quality 90+ is visually lossless.