If the photos are on the phone, then you just need to go into the "Settings" within the "Gallery" app and have it look for photos in the area that they are located. Note: This will only work if the photos are an image type that the "Gallery" can view. So .JPG files would be the best option.
If it isn't finding them, and it is looking for them on the device, then there may be a problem with the images. You would need to try and view them either through the phone's "File Manager" app or try connecting the phone to a computer via USB cord and make sure the files are indeed there and the correct format.