I have a Galaxy S3 and on it if you have successfully taken a picture look for it as a tiny picture on the lower left of the camera on the same line as the camera button and if you want to see that picture jut tap on it or use the Gallery application. I also installed a free photo editing program and it had no trouble finding all of the photos. My wife is not running the Samsung software but third similar firmware which our oldest son installed. For some reason it won't display all of the photos except in a photo editing program like "Photos", which is how I found all of her previously taken photos with no problems. In other words if you got the phone used it could be running other than Samsung's OS as her phone is, and in that case checking with the site of the firmware/rootkit software is the best choice.
Note that unless Galaxy S4 is a lot different than Galaxy S3, holding your finger down on the photo just focuses it on the spot where your finger is pointing. You have to use the onscreen button below the picture to take pictures.