Watermark

The picture is drawn on a canvas at its full size and the mark is painted over it, so what you download is one flat image: there is no layer, and the size and position you set are measured against the picture's own pixels rather than the preview on screen. Size is a percentage of the shorter side, which is why the mark looks the same on a small picture and a large one. Text is set in the same serif the page uses, with an optional dark edge behind it so that white lettering survives a pale background.

Tile across repeats the mark over the whole picture at the angle you chose. That is the honest kind of watermark: a corner mark is trivially cropped off, while a tiled one has to be painted out. Neither is protection. A watermark says who made something and makes casual reuse awkward; it does not stop anyone determined, and nothing that stays visible ever could — the page would rather say so than sell you the idea.

A logo is drawn from its own file at the same percentage size, keeping its proportions and its transparency. An SVG logo is drawn at the size it declares and scaled, so a logo with no width and height in it may come out small; a PNG with an alpha channel is the safe choice.

JPEG out re-compresses the whole picture; PNG keeps it exactly but is much larger for a photograph. Metadata does not survive the canvas, so the signed copy carries no EXIF or GPS. Both files are read in this tab, the page makes no network requests, and nothing you drop is stored; the Dark button keeps its own setting in this browser's local storage.