This article talks about modifying the EXIF data without the re-saving/re-compressing (and thus loss of information -- it actually uses a "trick"; there may be more direct libraries) required by the SetResolution
approach. This was found on a quick google search, but I wanted to point out that all you need to do is modify the stored EXIF data.
Also: .NET lib for EXIF modification and another SO question. Google owns when you know good search terms.