I don't know completely about the rules on here, as I'm quite new myself, but I can help you with uploading and posting your images though.
First of all, you go to
Image hosting, free photo sharing & video sharing at Photobucket (or any other imagehost you prefer) and you create an account. Afterwards, after you've saved the image on your computer, you press the upload button on Photobucket and select your signature/image.
When you've done that, you'll see, underneath the thumbnail, four lines of text/url. Copy the one that says "IMG Code" and paste it in the post where you want to show your image.
I hope I was clear enough.