Gifsicle

Resize a .GIF that exceeds Twitter's 15 megabyte limit

 * gifsicle --resize 320x400 --colors 256 2jgn6j.gif > new.gif


 * gifsicle --lossy=25 --crop 0,0+260x260 swing.gif > swing-cropped.gif

Extract the first frame of a GIF to a JPG file
gifsicle corgi-computer.gif '#0' > firstframe.jpg

Scale and merge

 * gifsicle --resize 500x375 -i dewey.gif > huey.gif
 * gifsicle --colors 256 --merge huey.gif cylon.gif -o robots.gif