Thick digit attaches to small, expandable images (10)

I believe the answer is:

thumbnails

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'small expandable images' is the definition.
Although both the answer and definition are plural nouns, I can't understand how they can define each other.

'thick digit attaches' is the wordplay.
'thick digit' becomes 'thumb' (I've seen this before).
'attaches' becomes 'nails' (nailing is a kind of attaching).
'thumb'+'nails'='THUMBNAILS'

'to' is the link.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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