Extract specific files in a tar archive using a wildcard

In the end, I found then answer after a good break. The option is wildcards, plural...

So the command

tar -xf MyTar.tar --wildcards "*.jpg"

did exactly what I needed.

Put quotes around the wildcard like this "*.jpg" so the shell won't try expanding it and will instead pass it straight through to tar. You want tar to evaluate the wildcard, not the shell and the quotes do that.