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[clarus]

How it do ? May be anybody have a plugin ... 

[Joachim Müller]

no plugin nor mod needed, it's a default feature of coppermine. Go to category manager, click the edit icon next to a category, scroll down and choose the thumbnail for the category from the dropdown list. Read the docs before posting.
Don't clutter the plugin board. All plugins we know are in the plugin contributions sub-board.
There's a difference between plugin, mods and core features. Moving your thread accordingly.