Ytdyklly

We usually write something like

includegraphics{image}

instead of:

includegraphics{image.png}  

That is, includegraphics does not usually require a file extension such as .png .pdf .jpg, etc... That is,

Suppose that I have all of the following files in the same directory. Which file will includegraphics embed?

my_pic.jpg

my_pic.psd

my_pic.png

my_pic.pdf

my_pic.jpeg

my_pic.gif

Document:

documentclass{minimal}

begin{document}

begin{figure}[htpb]

  centering

  includegraphics[width=linewidth]{my_pic}

   caption{Sunshine and Rainbows}

  Description{summertime}

end{figure}

end{document}

It depends on the engine. With pdftex the current list (which you can find in pdftex.def) is:

defGin@extensions{%

  .pdf,.png,.jpg,.mps,.jpeg,.jbig2,.jb2,%

  .PDF,.PNG,.JPG,.JPEG,.JBIG2,.JB2%

}

That means that e.g. pdf will be prefered, then png etc.

As documented in the graphics guide (section 4.5), this is user settable.

If you have

DeclareGraphicsExtensions{.pdf,.png}

in your document then it will just try .pdf then .png in that order, and ignore the rest.

The default value as shown in Ulrike's answer is set in the driver file for the option you are using. (Using a lower level def syntax rather than DeclareGraphicsExtensions because the driver files might be loaded by the color package, before DeclareGraphicsExtensions is defined.)