The paper "An Efficient Image Homomorphic Encryption Scheme with Small Ciphertext Expansion" (In Proc. ACM MM'13, pp.803-812) presented a novel image homomorphic encryption approach achieving significant reduction of the ciphertext expansion. In the current work, we study the security of this cryptosystem under a ciphertext-only attack (COA). We show that our proposed COA is effective in generating a sketch of great fidelity of the original image. Experimental results are provided to verify the validity of the proposed attack strategy.