Select the 'GIF resize' option at the top, select the GIF you saved and upload it.
Below the preview of your GIF it'll give you all the info you need. The height of your GIF must be less than 450 pixels. If it isn't, it's pretty self explanatory but all you have to do is put 450 in the height section although don't put anything in the width section as it will distort the image, leave it empty and it will re-size it respectively.
The size of your GIF must be under 1mb/1024kb - this may seem silly for me to put it in kb although when I did my GIF I was 3 kb over and it didn't work so be careful with that. If your GIF is larger than 1mb, you must 'optimize' it or it won't work. Select the 'optimize' option. Make sure you select the 'optimize' option that is with the second preview, not at the top, so it adds on to your 'resize' edit and doesn't start from scratch. I'd start with trying the 'lossy' option when optimizing it. This section is trial and error especially if you have a large GIF. Remember that you can stack the options i.e. you could use heavy lossy and also use colour reduction by choosing 'optimize' again in the second preview and simply choosing colour reduction in the drop down menu.
You're GIF should now be fine to use! Go to imgur.com and upload it. Copy the link with the .gif end to it that imgur gives to you when you open the picture e.g. http://i.imgur.com/h0XwqNQ.gif.
Last step. Head HEREand select the 'insert an image' option in the top toolbar. Paste the link of the GIF in and click update signature. You should now see a preview of your GIF. If you don't see it, there should be an error message in red at the top saying why it didn't work.