In the context of joint source and channel coding, unequal loss protection is often used to make image data more robust to possible packet losses. The allocation of source and code symbols is customarily done so as to maximize the expected PSNR at the receiver. We propose a new objective function, attempting to maximize PSNR given a constraint on the system probability of failure, so that PSNR is constrained to be above a given threshold with a given probability. This leads to the definition of a hybrid loss protection scheme, and the related allocation algorithm, which is able to satisfy this constraint. Experimental results are reported, related to the transmission of JPEG2000-compressed images over the Internet. It is shown that the proposed hybrid approach outperforms existing algorithms in terms of PSNR, while requiring less computational resources. © 2002 IEEE.