In order to deduce the orientation of the room, one would need the time and date that the picture was taken as well as the latitude of the location in question. The Sun moves from East to West everyday, that's true, but how North or South it will be of the location depends on the season. Half of the year, the Sun will be tracing paths North of the equator and the other half of the year the Sun will be tracing paths South of the equator. The Sun moves to a maximum of ~23 degrees North (Tropic of Cancer) and South (Tropic of Capricorn) of the Equator during the year.
So, for example, if the picture was taken during December at 12 Noon, then the Sun would be shining from roughly directly south of the location, so whichever wall the Sun is shining on then is the Southern wall of that room. If, on the other hand, the picture was taken at 8AM in June, then the Sun would be shining from NorthEast of the location and so the wall that the Sun shines on would the facing NorthEast. One could do some trigonometry to get the exact orientation of the house if one is given the exact time and the exact location (longitude and latitude), but for rough estimations of orientation, one probably just needs to know the latitude of the location, the month in which the picture was taken, and whether the picture was taken in the morning, noon, or afternoon.