Spectral distortions (SDs) of the cosmic microwave background (CMB) provide a powerful tool for studying particle physics.Here we compute the distortion signals from decaying particles that convert directly into photons at different epochs during cosmic history, focusing on injection energies E inj 20 keV. We deliver a comprehensive library of SD solutions that can be used to study a wide range of particle physics scenarios. We use CosmoTherm to compute the SD signals, including effects on the ionization history and opacities of the Universe. We also consider the effect of blackbody-induced stimulated decay, which can modify the injection history significantly. Then, we use data from COBE/FIRAS and EDGES to constrain the properties of the decaying particles. We explore scenarios where these provide a dark matter (DM) candidate or constitute only a small fraction of DM. We complement the SD constraints with CMB anisotropy constraints, highlighting new effects from injections at very-low photon energies (hν 10 −4 eV). Our model-independent constraints exhibit rich structures in the lifetime-energy domain, covering injection energies E inj 10 −10 eV − 10keV and lifetimes τ X 10 5 s − 10 33 s. We discuss the constraints on axions and axion-like particles that convert directly into two photons, revising existing SD constraints in the literature. Our limits are competitive with other constraints for axion masses m a c 2 27 eV and we find that simple estimates based on the overall energetics are generally inaccurate. Future CMB spectrometers could significantly improve the obtained constraints, thus providing an important complementary probe of early-universe particle physics.