What Is A Thumbnail Image?

A thumbnail is a small preview picture stored inside the file itself. It lets programs show you what a file contains without having to open and process the whole thing, which is why a folder full of photos or documents can fill with previews almost instantly.

Which files carry it and where the value comes from

Video files, photographs, raw camera images, vector graphics and documents can all carry one, and so can any other format that holds picture data worth previewing. Raw camera files often store more than one preview at different sizes.

What the preview shows depends on the file. For an image it is a miniature copy of the picture. For a video it is a still frame taken from the video stream. For a document it is normally the first page rendered as an image. Music files use the same mechanism to carry cover art.

The thumbnail is created by whatever wrote the file: the camera generates one as it saves the photo, an image editor rebuilds it when you save your changes, and an authoring application produces one for a document. Nobody chooses it by hand.

What the value looks like

A metadata report cannot print a picture in a table of text, so the row describes the data instead. The first part says what kind of data it is - almost always binary data, meaning raw bytes rather than readable text - and the second part gives its size:

• Thumbnail Image: (Binary data 8958 bytes, use -b to extract)

Nearby rows fill in the details, typically a Thumbnail Length in bytes, a Thumbnail Offset marking where inside the file the preview sits, and sometimes its dimensions. Embedded photo thumbnails are small by design and 160 by 120 pixels is a common size.

Why it matters

The everyday benefit is speed. Reading a few kilobytes of preview is far quicker than decoding a 40 megapixel photograph or a raw file, so browsing, sorting and choosing files stays responsive. The cost is a small amount of space: the preview is real data inside your file, and removing it makes the file slightly smaller.

The reason to pay close attention, though, is privacy. The thumbnail is a second, independent copy of the image, and it is not always rebuilt when the main picture changes. If a program edits the picture but leaves the embedded preview alone, the file goes out with a preview of the earlier version - the crop that removed somebody from the frame, the area that was painted over, the detail that was blurred out. The visible image looks correctly edited while the hidden one still shows what you took out, and anyone can pull that preview out of the file with ordinary metadata tools. The same applies to a document whose first page preview was made before the page was rewritten.

So if you edited a file specifically to remove something, do not assume the preview followed. Check that the thumbnail matches what you intend to share, or strip the metadata, which removes the embedded preview along with everything else.