A profile cover is a broad introduction to the account. It is different from a board cover or a single product Pin, so choose an image that represents the overall work rather than a narrow temporary detail. A studio scene, a coherent collection, or a finished project can provide context while the separate profile picture handles the account identity.
Pinterest's business help describes profile covers for business accounts, with a minimum of 800×450 and a recommended 16:9 shape. This page prepares our larger 1600×900 export in that shape. It is a working choice, not a claim that the larger dimensions are mandatory.
Use a source with enough room for a landscape crop. A tall poster may lose its main message when made wide, and a square product photo can become cramped. If the cover needs a designed arrangement of several images, make that composition in a design editor before using this tool for the final resize.