Graphics Gale, if you want free, use the free version. It's fully featured and won't run out, however, you can't load or run animations (.gif files).
Alternately, try out the trial for ProMotion. It's specifically designed for pixel art. It's about $80 or so, but as far as I know, the free trial won't run out of time. Just try both of these and see what you like. I would reccomend Graphics Gale.