Encryption is an important tool in many areas of application and research. The advances in communications have encouraged researchers to find new techniques for providing data security, confidentiality, integrity and authentication. The techniques proposed until now for image encryption apply well-known image processing techniques, increasing their computational complexity and processing time, threatening their use on real-time applications. On the other hand, the already proposed hardware implementations for image encryption do not allow portability to distinct FPGA platforms, and they do not guarantee high speed and optimal resource utilization. In this work, a generic real-time, FPGA-based, reconfigurable architecture for online image encryption using orthogonal functions is proposed. The introduced architecture implements a novel highly-efficient algorithm for high speed image encryption using minimal resources, which is portable to different FPGA platforms from distinct vendors. Obtained results demonstrate the effectiveness of the proposed approach on different cases of study, reaching processing rates up to 39 frames per second.