Healthcare institutions generate massive amounts of medical imaging data every day that need to be processed and stored. For this reason, large scale storage systems and processing power are required. To address this issue, cloud storage has been recently implemented in healthcare sectors. In fact, it provides scalable computational resources as a service. Furthermore, clients are charged based on their utilization of cloud services. In spite of its multiple advantages, moving medical data to an untrusted cloud storage provider arises security concerns. Besides, health information should be kept confidential and secret. To this purpose, we propose an approach based on the Shamir's Secret Sharing (SSS) method and multi-cloud environment to secure cloud-based medical image storage. In this paper, we apply the proposed technique to a gray-level image. The results of implementation show that our solution enhances data security by dividing the secret image into several shares. Consequently, the proposed approach is an appropriate method that allows healthcare organizations storing and sharing patients' medical information among healthcare actors in a secure and efficient manner.