Hi
The iMessages part if it has Aliases can have one set as the Send name.
New Messages that are then sent will then be from that alias
The are two possible ways that pick picks one account (AIM or Jabber) over another.
1) could be the order the Accounts are in the Messages > Preferences > Accounts list
This will also be the order in the Messages Menu > Accounts
And the Window Menu if you have it set to Separate Buddy List windows.
This will then effect the "From" name/account when you start with New Message in the Messages window then the Blue icon top right and chose the Buddies option.
The Buddies listed here are all the Logged in Buddies but will be based on the "first" Account first.
2) It could be the order in which that Buddy's various Jabber ID and AIM valid names are in their Address Card.
Or it could be which name/ID your Address Card features first.
If I type my Own name in the "To" spot then it seems to list the Info from my Address Card first based on the email listed first then Telephone Numbers then other AIM names I use.
However once I have selected which bit of Myself I am going to connect with then it is apparent that the "From" address is top AIM Name in the List.
9:56 PM Tuesday; October 30, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously