If you use an image editor like Photoshop or something more basic, you can
opt to choose the 'For Printer' option - this will copy the image to the
clipboard - get back to Photoshop - choose File | New (Photoshop
automatically chooses the new file size based on the dimensions of the image
within the clipboard) - then choose Edit | Paste and save to an image file
format.
You can save to a GIF file straight from Project as well.