It's a good song, very peaceful and refreshing, but there's one problem. Other than the image, there's nothing distinctively 'brony' about it. You could simply slap on a space image and you wouldn't recognize it as a 'brony' song.
All a signature is is the url of an image, so just get a pony, upload it to a site like imgur.com, then copy its url and put it in your signature between [img] or [simg] tags.