Location of Message Attachments in iOS 6 Backup?
Given your file:

/var/mobile/Library/SMS/Attachments/5D58C5EE-8F51-445C-A3FC-D6X94B6B6932/IMG_1230.png


Replace the var/mobile/ with MediaDomain- so that you end up with this:

MediaDomain-Library/SMS/Attachments/5D58C5EE-8F51-445C-A3FC-D6X94B6B6932/IMG_1230.png


Take the SHA1 hash of the above filename and that should be the name of the file in your backup: e6f11db0844899e5f8a04a796c74c234a3dcc5bc

To get the hash for testing, try the following link: http://www.fileformat.info/tool/hash.htm