First time I glanced at the title of this thread I thought it said... something else.
Did you enclose it in image tags?
Code:
[img]http://www.geocities.com/silver7allure/shimo_sig.png[/img]
If so, geocities may not allow that, or the forum may not allow .png format in signatures. Convert it to .jpg and throw it on imageshack.us then try again.