I use a program called picsaypro. Makes it easy.
You could technically use MS Paint. All u need to do is create a new one the size of your signature and while keeping it open, open another one to use to open your pics, cut, and paste. You'll just have to try it out. You'll get the hang of it.
Here's the catch, I use picsaypro on my phone (Droid) so just download the pictures or use my phone to take the pictures. Edit them and load the sig onto the site from my phone. 99% of the time I am on the site, hell all the internet, browsing from my droid phone.