I voted multiplayer - though the gui on the animations - if this is something you're planning could be integrated a little better.
Adding keys to skip back and forth between frames, deleting the blank frame when importing the sprite strip and perhaps a preview of the sprite all in the same window would be very cool.
<img src="http://fc06.deviantart.net/fs70/i/2012/097/2/0/gui_thing_by_johnnysix-d4vd59a.png" border="0" />
Ashley - ninja edit #3 - Would percentile scaling be possible? My maths is up to par as I work in finance, but it'd be a real timesaver if you could both lock proportional dimensions of height and width and be able to flip between pixel or percentile scaling.