If you want to do an attachement, go to the Additional Options at the bottom of the reply page and click on "manage attachments" button. There is a size limit on pictures, so you might have to resize your pictures.
If it is on a website, you can right-click on the picture, select "properties", and copy the Address/URL, then paste.
For multiple pictures, do you have any photo sharing account like photobucket, shutterfly or snapfish? If not, go ahead and open one. Then upload your pictures there and copy the url, then paste.