A key visual is a campaign's or brand's leading graphic motif – a repeatable composition of image, typography and colour that ties all communication materials into one recognisable whole.
While the visual identity sets the brand's permanent rules, the key visual is the "face" of a specific campaign, season or event. It's what makes a poster, a social post and a website banner instantly recognisable as parts of one initiative.
A good key visual: works across formats (from stories to billboards), leaves room for variable content, is distinctive enough to cut through the noise and disciplined enough not to fight the brand.
When we design key visuals for cultural institutions, we tie them to the identity system so each season feels fresh while the brand keeps its continuity.