Editing a GIF to make it smaller is quite some work
The best thing you can do is post the picture somewhere offsite on a picture site such as tinypic.com or imgur.com
Then you can open your picture in another window by right-clicking on it, copy the URL of the picture and then code it into the post which is easy!
Press the Img button that is above the reply window and you'll get this
[img]-[/img]
Now just paste your URL between the two
[img]URL-Here.gif[/img]
The result will be your picture, like this!