This is the sentence:
The image on the screen stares back at Susan with eerie intensity – it is a picture of herself
Microsoft Word would like me to say her, not herself.
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Picture nouns are a well-known special case for reflexivization, and possibly your grammar checker has special rules built in about that case. Do a web search for "picture nouns reflexivization" and you'll get lots of references. But no one understands reflexivization very well, and I certainly wouldn't accept the judgement of a grammar checker. In your example, both reflexive and non-reflexive forms sound okay, to me.