how to make a company logo to use on emails


Some one asked me how to add a company logo to all their emails. I know 1 email address in an AOL one but the others im not sure who they are with or what email client she is using.

I use incredimail as my email client and can change my background every email i send -which is why i was asked how i did it. although my server is VM as i use incredimail i can do all sorts of things.i very rarely go to the vm server for emails - only if im looking for something they might have stuck in the junk file - which my client never gets.
So does it depend on the server or the email client she is using to b able to change the background and put like a letter heading