In the era of telemedicine and cloud-based healthcare services, secure transmission of medical images, particularly DICOM images, is of paramount importance. This paper proposes a novel hybrid encryption methodology that integrates Ant Colony Optimization (ACO)-based watermarking with Advanced Encryption Standard (AES) to ensure both confidentiality and integrity of medical images. The ACO algorithm is employed to intelligently determine the optimal embedding locations for the watermark, preserving image fidelity while enhancing robustness against common attacks. The watermarked image is then utilized to dynamically generate a cryptographic key, thereby coupling the encryption process tightly with the image content and eliminating the need for external key storage. This content-dependent key generation not only strengthens the encryption mechanism but also enables tamper detection. Experimental evaluation using various metrics—such as PSNR, SSIM, entropy, NPCR, and UACI—demonstrates the proposed method’s effectiveness in preserving quality, attack resistance, and security.