Portrait cut-outs of well-known faces, trimmed to the edge of the hair and shoulders so nothing grey or blurred is left clinging to the outline. Fan pages, quiz slides and video thumbnails are the usual homes for these, where a face has to sit over a bold colour or beside a headline without an awkward rectangle behind it.
Do check how a particular photograph may be used before you publish it — likeness and photo rights rest with the photographer and the person pictured, not with the file format.