how to compare image file names in our project folder

Question

hi!
in my project i take images folder. i insert images that images are saved in my Images folder. my problem is when i insert same file name images means for example first i insert 1.jpeg and next i insert another image that file name is also 1.jpeg. this time first image is replaced with second 1.jpg. but my aim is first when i insert image it checks whether this filename is exits are not if exits give a message i.e., this filename is already exit. how can i solve this.
i take 
If imgThumbnail.ImageUrl = Server.MapPath("~/ProductThumbnails/") Then
Response.Write("This Thumbnail Filename is Already Exits. please Rename The New Thumbnail filename")
End If
 please help me. very urgent thank u sir.