How can I ftp a signature capture?

I'm using eb.signature, but can't seem to store the signature as an image.

Currently, I'm having to run a very crude script to save the data as a text file

1. stringify the capture as json

2. split the huge string into 10 hidden text form fields and pass to next page (otherwise too big to pass)

3. concatenate the string back together and save as a text file

It'd be much simpler to successfully FTP a png image to the server, but can't find much documentation on how this is done. The small amount of explanations I've found are very vague

Any help, or sample scripts would be great, thanks