I was trying to do something and i found what you need.
If the fragment shader, if you do :
float x = gl_FragCoord.x / the_with_of_the_viewport;
float y = gl_FragCoord.y / the_height_of_the_viewport;
gl_FragColor = texture2D(your_texture, vec2(x, y));
You’ll be good.
gl_FragCoord is the pixel position on the window. If you divide it by the with/height of the window, it will be in [0, 1]
So it will display your image it you use that as texture coordinates.