how can I share my pictures in iPhoto on my MacBook Pro to Pinterest?

Hello twins4me,

If you're looking to share photos from iPhoto, the best way to do so is via the Media Browser. The Media Browser is accessed by choosing to share or upload an item from within the app or webpage that you're using (such as Pinterest). Once the window for choosing the desired file opens you should see, on the left side, a category labeled "Media," which contains, among other things, the "Photos" item. Once you've chosen "Photos," you should then see the contents of your iPhoto Library, where you can choose items which you'd like to share.

iPhoto: Media Browser
https://support.apple.com/kb/PH19890

Cheers!