set image extension dynamically either .png or .jpg

how to set image extension dynamically either .png or .jpg because i have only image name in my database without extension in asp.net and in my folder there are .png and .jpg both files are available?

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the code which i am trying to set but if the particular file extension is not matched then how can i change .png to .jpg.

image.Src = "../image_data/" + dr["DesignNo"].ToString()+".png";