picture format change

Author: fpalacios

emails/SU2_6.0.1_Banner.png

@@ -193,7 +193,7 @@ <h3>The <font color="#B22234">S</font><font color="#3C3B6E">U</font><font color=
        </center>
 
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-<a href="https://su2code.github.io"><img src="https://su2code.github.io/emails/SU2_6.0.1_Banner.tiff" ></a>
+<a href="https://su2code.github.io"><img src="https://su2code.github.io/emails/SU2_6.0.1_Banner.png" ></a>