Given FreeImage will be trying to allocate it's own copy you're going to need double that. Also, seeing as compression would seem to be a completely pointless waste of time (as in why introduce huge delays when you obviously have the resources to manage uncompressed sizes) you would be better off IMHO to just save the raw image:
http://www.blitzbasic.com/codearcs/codearcs.php?code=1
http://www.blitzbasic.com/codearcs/codearcs.php?code=1