If you know how to load a file and make a usable texture from it then you are almost all the way there. To access a texture resource, call LoadBitmap. From there, follow similar steps to what you did with the file's texture. Both methods should result in an HBITMAP that OpenGL can use.
If that description is too vague then hopefully this sample app can help :
50 OpenGL Win32 Projects in One[
^]