Hi Vigour,
there is good and bad news I suppose.
I do ont know of a way of doing it with GD library. however, if you have access to your server setup, you could run the free php plugin from imagemagic, which can do it. Check out their website.
Altough I assume you cannot do this.. have you conisdered converting all images to jpg / gif before uploading them? Those formats can very easily be manipulated through GD.