There are three possible problems here. Two are your side of things and one is facebook's.
1. Image is too big. This may cause the image to be rejected by the uploader or cause a time out. Try using an image resizer to scale the image.
2. It's not actually an image you are trying to upload (it's rare but it does happen). Check you have a JPG, JPEG, GIF or PNG file.
3. Sometimes facebook has a problem with one of it's many servers - it is possible that the node (physical computer) dealing with your images has a fault and this will self correct or be fixed shortly. The solution here is to play a game or make a cup of coffee while you wait for things to sort themselves out.