Recently, an image encryption scheme using 2D Hénon-Sine map and DNA coding approach is proposed. It adopts the permutation-diffusion architecture. The DNA random coding and exclusive OR is introduced for image diffusion, while image scrambling is implemented by pixel swapping operations. This paper reveals that this encryption scheme is not as secure as declared. Substitution boxes (s-boxes) are firstly employed to summarize the involved complicated DNA encryption operations, the whole encryption scheme is subsequently relaxed as an s-box-then-permutation cipher. Chosen-plaintext attack is feasible for recovering the equivalent secret elements of the s-boxes and permutation vector. The proposed concept of generalizing DNA encryption as s-box substitution is expected to be beneficial for security evaluation and theoretical design of DNA-based image encryption schemes in the future.