How does one read or write files (png, txt, jpg...) in OpenGL ES? Target is Android through Visual Studio.
Unfortunately it's not as simple as placing the assets in the same directory as the main program and then referencing them using fstream.h or stdio.h like with the Opengl equivalent. I've tried creating folders like res/raw and using android/asset_manager.h and similar libraries. Is it even possible through this IDE? I'll be done in Unity by the time this gets resolved...
You don't. OpenGL is an API concerned with transforming vertices, and drawing pixels on a screen. File formats are outside the definition of OpenGL. In other words, if you want to use a *.png as an input/output format, you'll need to find a 3rd party library that supports that file format (e.g. libPNG), and use that to transfer the pixel data to OpenGL.
The raw file stream classes (e.g. ifstream) have zero concept of a file format. Again, another reason why you use a 3rd party library.
Unity is a full fledged game engine, and as such has spent time building support for various file formats (e.g. PNG, obj, etc). OpenGL is far lower level that that. A good place to start for image data, is a lib such as DevIL (which itself includes other 3rd party libraries such as libPNG, libJPEG, etc).