I've added experimental support for BLP1 files into MBNCSUtil. The support is not code-complete and so I'm not yet making an official release. However, you can check the files out of Subversion; details are
here. I haven't yet tested Jpeg compression - I just kind of implemented what the spec said - I haven't yet had a chance to track down a Jpeg-compressed version.
Here's a file from the downloads Ringo posted loaded into my test app:
Usage samples can be found in my test app, which is located under /branches/Blp1Test.
Note that repository code has the same version as the existing release, 2.0.5.18, and so certain dependencies may break if this is used next to production code.
Also corrected: an issue with the MpqFileStream class, in that it reported that it did not support seeking, but in fact did.