When working with Pages (and Word), it's hard to get the specifics of embedded images. If you want to know whether an image is a PNG or GIF or what, the interface doesn't make it easy, as pointed out on Betalogue.
I couldn't register to leave a comment (coughOpenIDcough) so I'm writing this here post to say: right-click on the Pages file containing the images you want to get at, and click on "Show Package Contents". All the images are available there as files for your perusal and eventual management. It's quite handy. Now if only they'd fix the damn file format.