If you want to select a folder then you use a FolderBrowserDialog. If you want to select a file then you use an OpenFileDialog or SaveFileDialog, depending on whether the file is a source or a destination.

Add your Button and OpenFileDialog to the form. Handle the Click event of the Button and, in the event handler, show the OpenFileDialog. If the user clicks OK, get the FileName from the dialogue and call Image.FromFile to create an Image object, which you can then assign to the appropriate form property.