Photo Aspect Ratio

Most of my photos have a 4:3 aspect ratio. When I enter multiple photos for a mural, the first photo is forced into a what looks to be a 16:9 aspect ratio and the top and bottom is cut off. The rest of the photos however retain their 4:3 aspect ratio and the entire photo can be seen. This also seems to be the case for other people’s photos. Why is the first photo forced into a 16:9 ratio? Can you make it so that all of the photos retain their original aspect ratio?

Art Rudick