PowerPoint 2010 does not import all of the pixels if the resolution of the image is greater than 150 ppi. It only imports enough pixels to create an image which is the size in inches times 150.
If you want to import all of the pixels and then be able to adjust the size of the image you have to first change the image resolution to something less than 150 ppi. Makes no sense, but that is the way it is.
Zimm

Many thanks for your reply, yes it doesn't make sense as my 'set default target output' of an image is set at 220ppi