You may not have fully understood https://stackoverflow.com/a/1369568/2375207 answer.
Define your North-West corner of the location on map: 51.5 °N, 5.4 °E This corresponds to the top-left corner of the picture, let's say x=0, y=0
Similarly with the South-East corner: 51.39 °N, 5.6 °E x=675, y=350
And just cross-multiplicate using the boundaries:
675 pixels <-> 5.6 - 5.4 = 0.2° longitude 350 pixels <-> 51.52 - 51.39 = 0.13° latitude