Revisited in 2015:
<img src="http://imageurl" style="width: auto; height: auto;max-width: 120px;max-height: 100px">
I've revisited it as all common browsers now have working auto suggested by Cherif above, so that works even better as you don't need to know if image is wider than taller.
older version:
If you are limited by box of 120x100 for example you can do
<img src="http://image.url" height="100" style="max-width: 120px">