Well I can think of a less than elegant way of doing it - not sure what it would do for image quality or anything like that - but see what you think; When you view the scanned image in preview or whatever you are using to see the several images in each file, now use Command+Shift+4 - if you're not familiar with this key combo - it will change your mouse pointer to a cross-hair - you then drag the mouse over the part of the screen you want to capture (in this case you drag the mouse over each separate image from the scanned file) When you let go of the mouse the area of the screen you have just captured is saved automatically to your desktop as a .png file - you can then (rename if required) drag the captured images to iPhoto - job done.
whoah whoah, hang on there. That way will give you photos appropriate for screen previews, and screen previews only. If it's a photo scanned at 300ppi(dpi) you will be losing a vast amount of detail.
There's a better way in Preview.
You can crop images in Preview. Go to Tools>Select Tool. Make a selection, and you can then choose to crop it (from tools) Then choose save as and select a new filename. The advantage to doing this - it will keep the true dimensions of the photo, and will give you more accuracy. Taking a screenshot won't allow you adjust the window dimensions. Once you release the click, the photo is taken. Cropping allows you to fine-tune the cropping window accurately.
The downside to this, is that you'll then need to reopen the original file to save out the next photo.
Alternatively, from the selection tool, you can choose File>New from Selection. This will create a brand new file to be named, leaving the original open, and ready for another selection to be made. Simple