www.photobucket.com
make an account and upload your image to that site, below it will have a IMG link so copy + Paste that in your signature. should look to be [.img] and [./img] without the periods.
If that isn't your personal site you can try
www.imageshack.com
or
www.tinypic.com
the results are anywhere but if you want it to be the quickest route, tinypic. If you want to establish an account where you can locate all of your pictures anytime, I suggest photobucket.