If you are the photographer of the photo, you own the copyright of the photo and hence can use it with or without attribution.
If you did not take the photo yourself, you can not use it (even if it is a picture of a national park). Because the person who pressed the button on the camera is the copyright owner of the photograph. The exception is if that photographer has given up his/her copyright by explicitly stating that it is "public domain", then you can use it. Or if he/she places it as "creative common", then you can use it provided that you obey whatever condition the photographer has stipulated -- usually the condition is that you have to attribute credit to the person who took the photo.
If you can not find any notice associated with the photo about whether it is public domain or creative common, then you have to assume that it is not. If you can not determine who took the photo, then you can not use it. All photographs are copyrighted by the photographer by default, unless otherwise explicitly stated. So if you do not see such explicit statement otherwise, it is copyrighted and therefore can not be used.