I had this problem before when I named homework sets with period in their name like 1.1, 2.1, etc.
This resulted in error message "Unknown graphics extension" .1-1-001.png with TeX confused about which period denotes the extension.
Adding the line
to the file hardcopyPreamble.tex
fixes that problem.
WeBWorK Main Forum
fix for periods in set names and png file names
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