The easiest way to do this in Mail is; next to the attachment field in Mail, you'll see Save & Slideshow. Use the Save pull down menu to "add to iPhoto" & the Slideshow is just what it says, it will do a slideshow of the photos in your email attachment.
I don't use Hotmail, but I'm sure all you do is click on the image, File>Save>Desktop. From there you can drag the image into iPhoto. Or simply drag the image from Hotmail to your Desktop or directly into iPhoto.
I hope that was helpful.