I reckon your question is whether you can get away with not providing the full path. If so, no. It's always best to provide the full path to the image.
You will find that sometimes it works okay even if you don't specify the full path, but don't depend on this behavior. The current directory, search directories etc. are not predictable at runtime, so you need to specify the full path always.
[Edit]
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In response to your comment, here's how you can get the path to the image folder in your application directory (using your example folder and image names).
string imagePath = Path.Combine(Path.GetDirectoryName(
Assembly.GetExecutingAssembly().Location), @"pic\pro.png");