(Try not to double-post, Star. You can update your last post by selecting the 'Modify' button at the top right of your post.)
And it may just be how you're coding the image. I suggest uploading your image (from your computer) to Tinypic.com (free, easy to use, safe to use) and copy+pasting the IMG code into your signature.
When you've done this, save, and then you should have a loverly signature of your choice. c: