Media placeholders are like windows or frames of a specific shape and size that you can drop photos or movies into. They overlay an image and hide or "mask" border areas of the original photo that you don't want visible. Yet you can still adjust the photo behind it by zooming in or dragging the photo around in the visible area of the window.
All the images in a Keynote or Pages templates are media placeholders that you drop photos into or copy and paste into another slide. You can also insert an photo this way. In the Format menu choose Advanced, then "Define as Media Placeholder". But it's easier to create a new one. Here's how.
1. First drop an image onto a slide or add it by clicking the Photos button on the toolbar. Don't worry how it looks now.
2. DOUBLE-CLICK the image. It looks the same but look below the photo and you'll see a tool bar. Be sure the CROP button to the bottom left of the image is blue (selected). * see #7 below if tool bar doesn't appear right away.
3. Initially, drag any of the sides of the original image in or out to the shape and size of the window you want. You'll see the excluded or "masked" areas of the photo dimmed behind the mask. I did a square in this example but you may be doing a rectangle.
EDIT THE WINDOW SIZE AND SHAPE (da puka) by clicking right on the window's border line to select it. Drag the sides or any of the Resizing boxes in or out and the window shape will change. This can be a little tricky - sometimes you click on the photo instead and it moves instead of the window edge. Just try again.
To POSITION THE PHOTO in the window click anywhere in the window and drag the photo around. Click the zoom slider on the bottom and drag the box to zoom in, and reposition again as desired. Note: You can zoom in to make a photo larger, but you cannot make it smaller than it's original size.
When done click anywhere on the slide outside of the photo and just the area in the window will be visible. The masked areas will be hidden. You can now drag the corners to resize the masked photo like a regular object.
To re-edit your placeholder DOUBLE-CLICK the photo to make the toolbar display again. You'll see the original photo with the masked areas dimmed. Make sure the Crop button is blue.
TO EDIT THE WINDOW: click on the window border line and drag the sizing boxes.
TO EDIT THE PHOTO: click in the center and drag, or drag the zoom slider. Click away from the image to see the result. At any point you can add a line or frame border or a drop shadow to your placeholder. You can even lower the Opacity in the Inspector Panel
Once you have a photo as a image place holder you can copy and paste it to each slide and just drop a new photo INTO the window area and it will replace whatever is there. Then just drag it to position it.