The sign-in graphic changed on my macbook air for no apparent reason.  what's up?

10-11  Source: Network gathering  Views:0 

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The sign-in graphic changed on my macbook air for no apparent reason.  What's up?  How do I get it back to my chosen graphic and why did this happen?
When I originally set up the computer, I just chose a preloaded picture that was offered.  (For example, I chose the sunflower, but now I get an eagle--but never chose the latter).  It's not really a big deal, I was just worried that somehow my computer might have been tampered with.  The OS update may be why it changed.
How do I change it back--if you wouldn't mind taking the time to explain something that is probably very simple to a not too computer-savvy user.  Thanks!
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