I couldn't find an Ada JPEG package but did find a public C one.  I
wrote an Ada wrapper, the decompressor side of which is just about ready
for alpha-test.  It's running under Windows so some "struct" rep clauses
might need to be changed for other systems.  The C code is under a "any
use OK, but mention where you got it" type of license, so the Ada
wrapper will be too.