A LaTeX Error Caused by the graphics Package — Argument of \Gin@iii has an extra } (Use graphicx Package Instead to Solve)

When writing blog¹, I found an error caused by the LaTeX graphics package:

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doc.tex: error: 10: Argument of \Gin@iii has an extra }. ...cludegraphics[width=\linewidth]{fig1.jpg}}

and I imported the graphicx package instead to solve this problem. These two packages are sort of different, although both of them are to display figures, and graphicx is preferred²:

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