Just hit “preview signature.” No need to publish a post to see it.
Don’t use the signature picture option at the bottom. Upload your picture to a hosting site like postimages.org or photobucket. Copy the direct link and then with your cursor in the body of text in your signature, hit the “insert picture” icon at the top of the window... it’s the yellow square icon. Paste your direct link in there and then hit “preview signature.”