SFML is an easy-to-use multimedia/graphics/windowing library. Great for learning the basics of event loops, user input, and basic graphics. It has noob-friendly guides for how to build it correctly. https://www.sfml-dev.org/
wxWidgets has a few quirks, but I've been able to use it quite well to create a GUI that surrounds an OpenGL panel w/ modern core context. https://github.com/wxWidgets/wxWidgets/tree/master/samples/opengl/pyramid
It seems to be more responsive (e.g. mouse events) than SFML for doing the same task, and I think this is because wxWidgets more closely uses the same event handling that Windows provides (you can directly override the WndProc function).
For newbies that feel installing & linking a whole library is too hard, LodePNG is a simple 2x file "library" for making PNG images that you can easily add to a project. https://github.com/lvandeve/lodepng
The GitHub page has examples. I used it a lot before now using stb_image (same library that SFML uses).