how to rename imae file

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C#2.0

C#

ASP.NET

hi friends,
Please anyone help me how to rename the image file..

1. The image file name is 1.jpg
2. I want to rename it according to the value entered in the textbox.

str = images[currIndex].Name.ToString();
           string name = txt_imagename.Text;
           File.Copy(str, name, true);

I tried the above code its not working.

Error:
Could not find file '1.JPG'.

please help me