Lossless compression and decompression functions are ubiquitous operations that have a clear high-level specification and are thus suitable as verification benchmarks. Such functions are also important. On the one hand, they improve the performance of communication, storage, and computation. On the other hand, errors in them would result in a loss of data. These functions operate on sequences of unbounded length and contain unbounded loops or recursion that update large state space, which makes finite-state methods and symbolic execution difficult to apply. We present deductive verification of an executable Stainless implementation of compression and decompression for the recently proposed Quite OK Image format (QOI). While fast and easy to implement, QOI is non-trivial and includes a number of widely used techniques such as run-length encoding and dictionary-based compression. We completed formal verification using the Stainless verifier, proving that encoding followed by decoding produces the original image. Stainless transpiler was also able to generate C code that compiles with GCC, is inter-operable with the reference implementation and runs with performance essentially matching the reference C implementation.