The file LatexTemplate.tex in ~/Library/TeXShop/Templates should contain the following lines if they are not already present:

  \usepackage{graphicx}
  \usepackage{epstopdf}
  \DeclareGraphicsRule{.tif}{png}{.png}{`convert #1 `basename #1 .tif`.png}

To make this change, find the appropriate template in ~/Library/TeXShop/Templates, open it with TeXShop, and edit as usual.

How do I configure TeXShop?
Default Latex Template