Have you tried refreshing the cache by turning it off in Admin => Global Options
As far as your edit of our file I can only say upload the original file and use it and follow the posts on Imagemagick here in the forums.
Try the edit noted here in bold in image-inc.php if the normal way does not work with Imagemagick otherwise GD2 will work fine for you cause Imagemagick can be a bear to get working on Windows in some configurations.
Code:
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