Okay! there might be few incidents when people message you even when you are invisible or blocked them by stealth settings.
Unfortunately there are many programs which allows you to detect invisble users on yahoo messenger.
For example there is a software called Buddy spy which worked amazing for some time but the new yahoo messenger changed few things in their server which made buddyspy to detect little difficult for example only the friends list could be detected using buddyspy. But later even that wast working.
But still there are few website which works wonder in the above terms.
They not only show their status message to you but also allows you to download their display picture which is irrespective of whether you are online or not.
Here is one of the popular website for detecting invisble users on yahoo messenger.
www.ydetector.com
Hence it is unsafe to put your personal pictures on yahoo messenger.
I hope yahoo authorities take few steps atleast to overcome the display picture problem or better not ;)