That's a shortfall with Cisco and the ASA. They have a security device and only one way to securely transfer files. And a poor one at that. Currently the only encrypted session is HTTPS.
http://www.cisco.com/en/US/docs/security/asa/asa82/command/reference/c4.html#wp2143863
We manage our firewalls via out-of-band devices. It has not come up yet, but I'm sure we'll have to figure out how to copy images through OOB and the ASA console.
Hope that helps.
