JPEG/JPG images have a format oddity that causes file pairs to be specific to each picture source rather than the file format in general. As an example, if you have pictures from two different cameras, and submit a file pair from the group of pictures from one of the cameras, then the decryptor will only be able to decrypt files from the camera that the file pair came from. Therefore, to decrypt all JPEG/JPG images, you will need to submit file pairs from every source you’ve obtained those pictures from.