Sprites and cropping

[Alric]

Two questions which I guess are fairly common issues:

I want to use animated sprites. So I have a bunch of images, one for each frame.
The size of the sprite varies as it moves (eg. if it reaches an arm out the image might get wider).
I want to crop the images to minimize their size, and compile them in a spritesheet (possibly).

There are plenty of tools to batch crop, and make a spritesheet.
But cropping will take an arbitrary numbe of pixels from each side of the image. This will move the handle by a potentially different amount each frame.
It seems like a simple task to write a cropping utility that saves the offset. But nothing I've found so far seems to do that.

Maybe it's not needed in which case I'm missing something. How do you deal with this problem?

Secondly, do I need to use power of two sized rectangles for the area I actally draw to the screen for a sprite (which would meantons of empty space)?