Last updated 2020/03/04
Some links from twitter on the topic of parsing PSD files (haven't looked into them in details). PSD include a flattened rasterized image so this is easy to load if that's the only thing you need. Most software / librairies have support for extracting this flattened data (e.g. stb_image.h does).
However if you want access to individual layers, render non-rasterized layers, emulate every photoshop features, extract or apply effects with more granularity, more code is needed. May range from easy to lots-of-work depending on what exactly you need.
As far as I know there isn't a trivial stb-like ready-to-use C++ library to do that sort of things. Posting all links here. Some are probably bloated or hard to use into your project, some lacking features.
TODO: Actually look into the pros/cons of all those.