how to create gif animation from a stack of jpgs

Try using ImageMagick's convert utility. I have used it to create animated gifs from a set of images (in any format) in the past.

Use the command

convert -delay 20 -loop 0 *.jpg animated.gif

If you'd like a flexible on-line solution, I just used GIFmaker.me and it worked great. It lets you change the frame order, change the size, set the speed, and set the repeat cycles. You can view the animated GIF and download it when you're finished.

Edit: I just used another on-line tool that GIFmaker refers to on their site. GIFcreator is even more flexible, letting you duplicate frames, change the delay for each frame, remove frames, and reverse frames. It also has a more flexible resize capability.

In theory this would work

ffmpeg -f image2 -i image%d.jpg video.avi
ffmpeg -i video.avi -pix_fmt rgb24 -loop_output 0 out.gif

Might want to look at GiftedMotion: http://www.onyxbits.de/giftedmotion