Here is the simplest way to save a graphic to your computer from an email that arrives in Mac Mail with a picture in the message:
Hold Down the CONTROL key and click the mouse key or trackpad key.
A popup menu shows a list of possible commands … the best being SAVE TO DOWNLOADS FOLDER. If you select SAVE TO DOWNLOADS FOLDER, the graphic is saved to the hard drive in the Mail Downloads folder and that folder comes to the front of the display and you should see the graphic file icon in the folder.
The next step will make this graphic the one and only graphic in your LAST ROLL in iPhoto, so make sure you have organized your iPhoto LAST ROLL before you do the next step … Next, you can drag the recently downloaded file into the iPhoto icon in the Dock (the row of icons at the bottom of the Mac desktop) … this automatically opens the graphic after importing it into the iPhoto application. You can then organize this photo into a folder in iPhoto just like you would organize a photo or a group of photos from a camera import.