1. Right-Click on the steam gif.
2. Choose save-picture as.
3. On the pop-up window, click save.
3a. (optional) If you would like to rename it, rename it anything you wish, but it MUST be ending in .gif
4. Upload the .gif to a picture hosting site such as imageshack.us
5. Take the forum code after uploading and paste it into your signature, located under User CP, Edit Signature.
5a. (optional) If you cant find the FORUM code, take the first code (direct link) and paste it in. Then in front of the code type [IMG] and after the code [/IMG]