OpenGL, unlike Direct3D, is operating system independent. While this eases the portability among different OS, it implies that we have to resort to external libraries for a number of tasks, loading images being one of the most used.
There are many libraries to load images, DevIL being one of them. It loads a large number of image formats and it is cross platform.
A short tutorial for loading an image, getting its data and attributes, and finally creating an OpenGL texture is available in here.