As something that always hangs on the wall, it is common for photo frames to catch dust and look faded after every few months. To keep the pictures looking good all the time, you should dust them often. But if you happen to let the dust stay there for a long time, then it will take a bit more thorough cleaning to restore the appearance of the frame. In that case, you can wipe the frame with a clean and slightly damp cloth. Spray some glass cleaner on the cover and wipe it off. When you are cleaning, make sure that no moisture gets inside the frame.