Are you using the images in JPEG format? These warnings look like corrupt exif metadata in the JPEG file that the function is trying to read. This should not affect the actual pixels, so this warning can be ignored. However, please note that saving the calibration images in the JPEG format is not a good idea, because of the compression artifacts. If possible, consider using a format with no compression or with lossless compression, like TIFF or PNG.
Hope this helps.