You can make high quality animated GIF images online for free with Imgflip, using simple and powerful customization options.
Some of the popular supported video formats are flv, avi, mov, mp4, mpg, mpeg, wmv, 3gp, asf, swf, ogg, h264, rm. But there are hundreds more...
Imgflip Pro lets you use videos up to 200MB! If you're only using a few seconds of a really long video, you can try to find a shorter version, or just use a computer program to chop up the video.
The Imgflip watermark helps other people see where the GIF was created. It has been made as small as possible while still being readable, and it will not even show up on tiny GIFs. You can disable the watermark completely with a Pro account, if you really need to have watermark-free images.