A method for decoding an image according to the present document may comprises the steps of: deriving a modified transform coefficient; checking whether the tree type for a current block is a dual tree chroma; determining whether an MIP mode applies to the current block on the basis of the current block not being the dual tree chroma; determining whether an LFNST application width and an LFNST application height corresponding to the current block are 16 or greater, on the basis of the MIP mode being applied to the current block; and parsing an LFNST index on the basis of the LFNST application width and the LFNST application height being 16 or greater.