I have never encountered an error on style images; out of curiosity, what're your system paths set to (CP -> Master Settings -> Paths & Database -> Absolute Path to Forum Directory & Absolute Path to Sessions Directory)?

You can remove the check entirely by removing the file from the /install/UPGRADE_CHANGES.txt and then click retry (line ~226):
Code
VERSION-7.7.4:NEW: styles/images/chalkboard.jpg

But I'd love to see your paths to see if it could be triggering the warning.

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