Baseline JPEG stores images with 8 bits per color sample, in other words
24 bits per pixel for RGB images, 8 bits/pixel for grayscale, 32 bits/pixel
for CMYK, etc.  There is an extension that stores 12 bits/sample for
applications that need higher accuracy.  Medical images, for example, are
often 12-bit grayscale.  The 12-bit extension is not very widely supported,
however.  One package that does support it is the free IJG source code (see
part 2, item 15).

For lossless JPEG, the standard permits any data precision between 2 and 16
bits per sample, but high-precision lossless JPEG is even less widely
supported than high-precision lossy JPEG.  The Stanford PVRG codec (see
part 2, item 15) reportedly supports up to 16 bits/sample for lossless JPEG.