Only half a photo will print from iPhoto

What is the aspect ratio of the photo?

Have you first cropped the photo to 4x6?

Have you selected 4x6 borderless as the paper size in the iPhoto print window?

Have you selected Borderless as the print size?

If you've done all of that and still are getting a problem try this: make a temporary, backup copy (if you don't already have a backup copy) of the library and try the following:

1 - delete the iPhoto preference file, com.apple.iPhoto.plist, that resides in your

User/Home()/Library/ Preferences folder.

2 - delete iPhoto's cache file, Cache.db, that is located in your

User/Home()/Library/Caches/com.apple.iPhoto folder.

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3 - launch iPhoto and try again.

NOTE: If you're moved your library from its default location in your Home/Pictures folder you will have to point iPhoto to its new location when you next open iPhoto by holding down the Option key when launching iPhoto. You'll also have to reset the iPhoto's various preferences.

OT

Almost any image will print correcty when printed on paper that is larger than the image itself. It's when you want to print that image on smaller paper that you must crop the image to the same size as the paper selected.

If the image is not the same aspec ratio as the paper stock printed on some of the image will be cropped out. That's just how printing works. One can not squeeze the pixels in an image that have a 1: 1.4 size ratio into a 4x6 print. There are too many pixels and some have to be cropped.

Try logging into another user account on your Mac and print from there. Have you gone to Canon's website and downloaded the most recent driver from there and used it? That might jump start the process.