How to Get Complete URL using Disk File Path

Consider photosLocation as a path relative to the application; for example: "~/Images/". This way, you could use MapPath to get the physical location, and ResolveUrl to get the URL (with a bit of help from System.IO.Path):

string photosLocationPath = HttpContext.Current.Server.MapPath(photosLocation);
if (Directory.Exists(photosLocationPath))
{
    string[] files = Directory.GetFiles(photosLocationPath, "*.jpg");
    if (files.Length > 0)
    {
        string filenameRelative = photosLocation +  Path.GetFilename(files[0])   
        return Page.ResolveUrl(filenameRelative);
    }
}