Are you talking about the user icon? You should be able to select it from System Preferences-->Accounts. Once there, click on your account name and select the Picture tab. Once there, click Edit and a window should popup. Click Choose and then pick your picture.
As for QuickBooks Pro, I couldn't help you there as I have never used it. Hopefully someone here with experience on that can help.
Oh, and welcome to the forum and the Mac.
