I agree. A simple cleaning with soapy water
may be all that's needed. Use a soft sponge
or soft cotton cloth. Rinse well with clean
water on the sponge or rag after soaping.
Don't overdo the wetting - be sparing in your
applications.
Depending on when the painting was varnish, you might be able to remove the
varnish using turpentine. If turpentine doesn't work, try toluene but remove
the varnish very carefully. Soak a Q-tip with toluene and work on the drip
lines first. If you remove any paints, stop. If conversely the toluene can't
remove the varnish, you are in trouble - some idiot probably saturated the
varnish with a drying oil, which is why sometimes varnish yellow. In this case
you need to take the painting to a professional cleaner who will charge you a
princely sum.
Hope this helps.
Dik