How to add .jpg to end of sting if it doesn't exist

I have the "original list" and I'm trying to convert it to the "desired list" using php. When the .jpg is present at the end of the string, I want to do nothing. If .jpg isn't the end of the string, I want to add .jpg to the end.

   Original List                                      Desired List
http://ift.tt/2hyMt3a                    http://ift.tt/2hBPrB6
http://ift.tt/2hyWwFx                http://ift.tt/2hyWwFx
http://ift.tt/2hBPahE                    http://ift.tt/2hyWGNb
http://ift.tt/2hBKgkZ                    http://ift.tt/2hyVIAI
http://ift.tt/2hBF8Ns                http://ift.tt/2hBF8Ns

I've been using the following to modify a strings if someone added a , where a . belongs, but now I need to modify the sting only if the string doesn't end in .jpg.

$badPhotos = str_replace(',', '.', $badPhotos);

Ideas? Thanks!