it's for windows, it's free and very powerfull.
You can make it do exactly what you want.
Take some time to see how it works.
Oh yeah, originaly it was a command line utility from Microsoft, and some people put an interface on it.
|Waffles with fruits by Coolinarka|
from Food photography (desserts)
|Vestrahorn Frozen Reflection by Will B Milner|
from Ice cold