SuiteCRM Access to Theme Files

This is likely a basic question and I apologize in advance if it’s overly simple. I’m trying to add a custom image file to a PDF Template but am not seeing an option to upload an image, only to insert an image that is already available on the server. My expectation is that I would be able to utilize SSH, FTP or some other utility to access the underlying SuiteCRM files, including themes, but haven’t been able to locate them. Would someone be able to point me in the right direction on this? Thank you in advance.