The common method to display a full sized image is to use an orthogonal projection matrix:
(example using fixed functionality)
GLES20.glViewport(0, 0, windowWidth, windowHeight);
GLES20.glMatrixMode(GL_PROJECTION);
GLES20.glLoadIdentity();
GLES20.glOrtho(-1, 1, -1, 1, 1, -1);
than draw a fullscreen quad with your image bound as texture2D.
basic texture tutorial here.
basic quad tutorial here.
set quad coordinate as follow:
static float squareCoords[] = {
-1f, -1f, 0.0f, // top left
-1f, 1f, 0.0f, // bottom left
1f, 1f, 0.0f, // bottom right
1f, -1f, 0.0f }; // top right