If you already have your digital photos stored in iPhoto it's easy to import them to your iMovie project. Click on Media in the bottom right corner to open the media menu, then on Photos in the top of the menu.
Your iPhoto library will open right inside the iMovie project. You can browse through your pictures and drag the one you want to use to the timeline.
If your photos aren't saved in iPhoto you can access them by clicking File > Import... This will open a browsing window that you can use to find your picture files.