What is the yellow warning sign in the "Login Items" Tab under the "Users & Group" System Preferences?
The yellow warning sign means that the login item can no longer be found, most likely because the application was deleted or moved after it was added to the login items. If you hover your cursor over the triangle, it will tell you:



To re-add iTunesHelper:


Find iTunes in /Applications
Right-click on it, and select "Show Package Contents".
Navigate into Contents, and then MacOS
Drag iTunesHelper into your Login Items in System Preferences
Check "Hide" to hide it on launch (this may not be necessary, but it's how iTunes sets it up)