This won't be helpful, but the same effect occurs when you are uploading files using the asyncfileupload tool.  Any transparent backgrounds that go into the "file upload/resize before saving" come out with those areas that were originally transparent turned to black.  In all truthfulness, I do not believe this to be a Telerik-related issue.  I have found hundreds of forum entries out there describing this same result regardless of the tools used.  It appears to be more Windows related.