Control Panel -> Help and Support -> How to replace a thumbnail for a file
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How to replace a thumbnail for a file

You can upload your own custom thumbnails if required. This changes the display thumbnail only - you will not make any changes to the original file. Typically this feature is used to provide a more meaningful thumbnail for non-image file types like ZIP and PDF and illustrator files. Controlled from the file information page (Control Panel > My Images > Click on the collection you wish to edit > click on the file you wish to edit, then use the Replace Thumbnail icon).

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