I use MacTex, and my editor is TexShop. It probably has to do with what compiler you are using. When I use pdftex, the command:
\includegraphics[height=60mm, width=100mm]{number2.png}
works fine, but when I use "Tex and Ghostscript", I get the same error as you, about not being able to get the size information. Use pdftex.
Incidentally, you can change this in TexShop from the "Typeset" menu.
Hope this helps.